Topic: Tracks and signalling

[thegamer7893]

From my understanding from my work, differential speed limits are usually related to signalling (although there are other cases such as tilting trains on the WCML and some structures). Generally they exist to allow a stock type to use a line for which the signalling has not been designed, or to get a bit more of an advantage out of rolling stock which has better performance than the current system. My experience of working on projects with multiple signalling systems is that differential speeds tend to be far more common in areas with 4 aspect signalling than 2 aspect or absolute block signalling.

Traditionally a line's signalling is designed with the braking distance of the slowest braking rolling stock in mind, which usually will be freight. However on busy commuter lines, such as the Brighton Mainline (BML) freight is relatively rare and designing the signalling to accommodate freight would comprise the headway of the line for little to no benefit. Therefore to enable freight to use the route a differential speed limit is created (BML uses the 2/3rds rule which states that freight trains are permitted at 2/3rd of the linespeed except where a specific differential speed limit applies). This speed limit will be calculated such that the slowest braking speed for any permitted freight consist will allow it to stop within 1 or two signalling sections (depending on the number of aspects that signalling system uses).

The opposite can also be true. In some cases a train may have a slightly better braking characteristics then the rolling stock that the signalling was designed for. As such certain stock types (usually multiple units or HSTs) can be permitted at a speed limit higher than line speed (normally only 10 - 15 Mph higher) in order to achieve slightly improved journey times. Such speed profiles were commonly introduced by BR in the 1980s when HSTs and Sprinter DMUs began operating on lines previously worked by loco hauled trains. In this case it was not economical to upgrade the signalling for the new rolling stock, but journey time improvements could be made by increasing the speed limits for the new stock to operate within the existing system.


This picture shows an example of a differential speed profile with a higher speed for DMUs (albeit with a non-standard sign)

It is possible that some features of the Permanent Way will require a differential speed profile, however from my experience this is not usually due to track curvature and more to do with weak structures. Freight trains exert considerable dynamic loads when travelling at high speeds and as such may be slowed to reduce the wear on certain structures on a route.


For example this extract from the National Electronic Sectional Appendix (this is available publicly by the way) shows the famous Morpeth curve at the top of diagram where the speed limit changes from 110 Mph to 50 Mph. Despite this being one of the sharpest corners on a mainline in the UK there is no differential speed limit as the signalling system can accommodate freight trains at line-speed.

From an in game perspective a lot of this should be represented by the signalling modelled in the game, however a useful feature could be to allow the player to chose a line-speed for a section of line. Often when express trains arrive into termni in cities it could be useful to restrict them to the speed of local trains to prevent them catching up too quickly and therefore constantly stopping and starting when they catch up with a slower train, the same can be true of freight trains when they are behind a stopping service, a lower speed limit for freight could reduce the need for them need to stop completely.

Taking this quote from another topic which I started a couple months ago. I've been looking at this reply that James got from SuperTimo and I found this very interesting. Because, I directly think that like higher permissive speed limit tracks should be a feature on Simutrans Extended and that weight/tilt restricted tracks should feature in-game to give people an option of running loco-hauled passenger or freight trains without the need to use track with a higher max, speed.

[thegamer7893]

No replys yet?

[Vladki]

OK, let me sum up what do we already have in simutrans:

Signals have speed limit that should be relevant to their visibility (e.g. dwarf, classic, cab signals).
Every way/track has a speed limit, and axle load limit; bridges also total weight limit. You can change the behavior in simuconf.tab:

Code: [Select]

# This setting allows the way in which weight limits are enforced to be customised.
# The meanings of each setting are as follows:
# 0: weight limits are ignored entirely.
# 1: then vehicles can travel over roads, rails, tunnels and bridges for which they are too heavy, but only at greatly reduced speed.
# 2: Vehicles cannot travel over ways for which they are overweight at all.
# 3: Vehciles cannot travel over ways for which they are more than 10% overweight at all, and will travel on ways for which they are up to 10% overweight at a greatly reduced speed.
#
# To revert to the behaviour of Simutrans-Standard, set enforce_weight_limits=0

enforce_weight_limits=3
Although I don't know what exactly is the "greatly reduced speed".
And we have tilting trains, that can pass curves 30% faster faster.

And what is in real world (Czech railways).
There are different speed limits for:
1. tilting trains - higher speed in corners (cca 35%)
2. 6 axle engines (Co'Co' or 3-3) - lower speed in corners (due to higher wear)
3. selected non tilting trains - slightly higher speed in corners (cca 10%) - perhaps those with better brakes and suspension, multiple units, with possible exceptions for given track,
4. cargo trains - lower speed limit only on some places, specially mentioned in track info.

Example here: https://cs.m.wikipedia.org/wiki/Soubor:Rychlostniky_Chocen.JPG
The basic speed limit is 80, some engines can go 85 (#3), 6-axle engines 70 (#2), and tilting trains 100 km/h (#1, the tilting speed sign is further back at the signal)

Out of those, we have #1 already implemented. #2 and #3 could be made as generalization of #1. Instead of binary value (tilting or not), could be a % that says how much faster or slower this train can go through corners.
#4 is imho for places where train weight matters like old bridges that do not match the axle load of the rest of the track. This is possible with setting enforce_weight_limits=1.
So far none of these are relevant to signalling - only to track geometry (curves), axle load, and train characteristics.

For signalling there is one important value: braking distance. It is a distance at which train should be able to halt. It of course depends on speed, and is defined:
400 m for speeds < 60 km/h
700 m for speed 60 - 100 km/h
1000 m for speed 100 - 120 km/h
1000 m for speed 120 - 160 km/h with cab-signalling
(higer speed is not allowed in CZ)
All distant signals and 3-aspect signals are built at this distance from next signal. If a train is not capable to break at this distance, it must reduce its speed accordingly. This is partially done in simutrans too. If you put the distant signal too close to the main signal (or not at all), trains will start braking earlier even if the signal is clear. Moreover, trains without cab signalling are limited to 120 km/h. So it seems that the 1 km distance is not enough for higher speeds, and cab-signalling gives you one more block (1 km) to brake. Of course trains do not have to start braking immediately after passing caution signal. The driver has to know how much he needs with given train. (That is also implemented in simutrans). 4-aspect signalling was used to increase capacity by using blocks shorter than braking distance (cca 1970-2010), now it is used only in stations, when you need shorter blocks due to track layout. Again 4-aspect signaling works well in simutrans.

So I think simutrans-extended has all this covered quite well. What exactly are you missing?

[thegamer7893]

I am just trying to explain how like there are tracks which is limited for certain trains (i.e. on the Chiltern mainline) where past High Wycombe, there is 1 55/85mph limit the 55mph limit applying to freight trains and the 85mph limit applying to passenger trains (including those loco-hauled). So, in simple terms. I'm asking for a bit of track variation within the game as like there are also lines which have speed limits for heavier trains (i.e the Middlesbrough line) where there are speed limits which apply to the class 185s which are slower due to there weight and faster speed limits which apply to sprinter trains.

[Vladki]

As I said above, would not that be the case for enforce_weight_limits=1 or 3 ?
In that you can let overweight (freight) train on cheap track, but it will go slowly...

[thegamer7893]

Yeah, thats the issue you see and I want to run loco hauled pass. trains but with the weight restrictions it seems impossible as they will run slowly.

[ACarlotti]

where past High Wycombe, there is 1 55/85mph limit the 55mph limit applying to freight trains and the 85mph limit applying to passenger trains (including those loco-hauled)

The 55/85 differential limit is due to different braking rates - the line mostly uses 3-aspect signalling which wouldn't allow (many) freight trains enough time to stop safely after encountering a yellow 'caution' signal. That line also has some locations where the speed limit differs for variants of the same class of DMU, depending on whether it has end gangway doors or not, due to door restricting visibility of signals in certain locations.

The way that Simutrans (Extended) handles this sort of speed restriction is different to how it is handled in reality. Firstly, Simutrans estimates the braking rate of a train and uses that to work out how fast it can go through a signal. This is a much more precise restriction than in reality, and allows each train to effectively have a different speed limit. Secondly, the speed limits in reality are generally chosen to be speeds at which trains can drive continuously without having to slow down until a restrictive signal aspect is seen. On the other hand, in Simutrans trains will quite happily run faster than this between signals, but will then brake on approach to signals until the driver can see that it has a clear aspect, at which point they will accelerate again. This behaviour is probably easier to program an use, but woudn't be applied in reality because it would have a significant detrimental effect on energy consumption.

(i.e the Middlesbrough line) where there are speed limits which apply to the class 185s which are slower due to there weight and faster speed limits which apply to sprinter trains.

As far as I can see (looking at the Sectional Appendix) there aren't currently higher permitted speed limited for sprinter trains on the Northallerton-Middlesborough line, despite what Wikipedia suggests. However, they do exist elsewhere, and the principle is still relevant. Perhaps what you want is a gradual reduction in speed limit for trains with an axle weight higher than that normally permitted - maybe something like 50% higher weight leading to a 40% reduction in speed limit, with anything exceeding this prohibited altogether. (I've just plucked those numbers out of thin air - better numbers would be required if such a feature were to be implemented.) 

[thegamer7893]

The 55/85 differential limit is due to different braking rates - the line mostly uses 3-aspect signalling which wouldn't allow (many) freight trains enough time to stop safely after encountering a yellow 'caution' signal. That line also has some locations where the speed limit differs for variants of the same class of DMU, depending on whether it has end gangway doors or not, due to door restricting visibility of signals in certain locations.

The way that Simutrans (Extended) handles this sort of speed restriction is different to how it is handled in reality. Firstly, Simutrans estimates the braking rate of a train and uses that to work out how fast it can go through a signal. This is a much more precise restriction than in reality, and allows each train to effectively have a different speed limit. Secondly, the speed limits in reality are generally chosen to be speeds at which trains can drive continuously without having to slow down until a restrictive signal aspect is seen. On the other hand, in Simutrans trains will quite happily run faster than this between signals, but will then brake on approach to signals until the driver can see that it has a clear aspect, at which point they will accelerate again. This behaviour is probably easier to program an use, but woudn't be applied in reality because it would have a significant detrimental effect on energy consumption.
As far as I can see (looking at the Sectional Appendix) there aren't currently higher permitted speed limited for sprinter trains on the Northallerton-Middlesborough line, despite what Wikipedia suggests. However, they do exist elsewhere, and the principle is still relevant. Perhaps what you want is a gradual reduction in speed limit for trains with an axle weight higher than that normally permitted - maybe something like 50% higher weight leading to a 40% reduction in speed limit, with anything exceeding this prohibited altogether. (I've just plucked those numbers out of thin air - better numbers would be required if such a feature were to be implemented.)

Oops, I think I made a mistake with what line it was. It was the Scarborough line btw.

[thegamer7893]

And just for some reference, this is where I kind of got what I kind of want in-game from: https://www.youtube.com/watch?v=Z7K43e5uwKc

[Vladki]

That is quite long video. What part is interesting?

As said before, if the limit is due to braking, then that means that the blocks are too short, and simutrans will already make the trains run slower (break earlier than at the signal). And that should force you to fix the signalling - either use 4-aspects, or make longer blocks. Also in real world, when train is assembled, its weight and braking capabilities must be calculated and compared with the tracks requirements - steepness of downhill parts and block length (or distance of distant signals), and the top speed might be limited if necessary. That is done by simutrans automatically, as if the driver always knows where the main signal is and know when to start braking.

This is interesting topic and made me study more about railways. In CZ freight trains are generally limited to 100 km/h - this can be enforced in the dat file for wagons. Only some can go 120 km/h when loaded, some others can go 120 km/h only empty. Every freight wagon has a table stating how much you can load it (at 100 or 120 km/h) in order not to go over the allowed axle load for given class of track.     A = 16 t/axle; B = 18 t/axle; C = 20 t/axle;  This makes it quite complicated. Part of that would be nice to have in simutrans. If the track (or road in general) to the destination is weak to support fully loaded vehicles, load them only partially so that they can pass. That would be nice addition to enforce_weight limits.

Also tracks support different axle loads at different speeds. As the freight trains are limited to 100 km/h, the weight limits usually apply to this speed. Passenger trains are usually lighter (and break faster) so they may go faster.  For example, Concrete sleepers B03 (252 kg) with rails S49 (49 kg/m) support 22.5 t/axle at 120 km/h but only 18 t/axle at 160 km/h. It might be nice to be able to specify set of weight/speed parameters for single way type.

To sum it up I think we could have these improvements:

- more pairs of weight/speed limits for each way
- instead of tilting 0/1, have a % how faster or slower can given train go in curves than "normal train"
- allow vehicles to load only partially in order to use weaker ways

However I do not see these as really fundamental problem.

[thegamer7893]

That is quite long video. What part is interesting?

As said before, if the limit is due to braking, then that means that the blocks are too short, and simutrans will already make the trains run slower (break earlier than at the signal). And that should force you to fix the signalling - either use 4-aspects, or make longer blocks. Also in real world, when train is assembled, its weight and braking capabilities must be calculated and compared with the tracks requirements - steepness of downhill parts and block length (or distance of distant signals), and the top speed might be limited if necessary. That is done by simutrans automatically, as if the driver always knows where the main signal is and know when to start braking.

This is interesting topic and made me study more about railways. In CZ freight trains are generally limited to 100 km/h - this can be enforced in the dat file for wagons. Only some can go 120 km/h when loaded, some others can go 120 km/h only empty. Every freight wagon has a table stating how much you can load it (at 100 or 120 km/h) in order not to go over the allowed axle load for given class of track.     A = 16 t/axle; B = 18 t/axle; C = 20 t/axle;  This makes it quite complicated. Part of that would be nice to have in simutrans. If the track (or road in general) to the destination is weak to support fully loaded vehicles, load them only partially so that they can pass. That would be nice addition to enforce_weight limits.

Also tracks support different axle loads at different speeds. As the freight trains are limited to 100 km/h, the weight limits usually apply to this speed. Passenger trains are usually lighter (and break faster) so they may go faster.  For example, Concrete sleepers B03 (252 kg) with rails S49 (49 kg/m) support 22.5 t/axle at 120 km/h but only 18 t/axle at 160 km/h. It might be nice to be able to specify set of weight/speed parameters for single way type.

To sum it up I think we could have these improvements:

- more pairs of weight/speed limits for each way
- instead of tilting 0/1, have a % how faster or slower can given train go in curves than "normal train"
- allow vehicles to load only partially in order to use weaker ways

However I do not see these as really fundamental problem.

I mean ideally, I want there to be a feature where you as the player can set the speed limit of that line (track) so that in certain places. You can set the speed limit to a lower speed limit for certain places (i.e. terminus stations). But, I do understand where you are coming from.

[jamespetts]

Thank you for your feedback.

It is not clear what advantage that there would be to a player in setting a speed limit on a piece of railway line: a player always has an incentive to allow trains to travel as fast as possible. In reality, people who run railways set speed limits because, without them, the trains might crash because they cannot stop for signals in time, derail on corners or at least the passengers might be thrown around inside the trains travelling too fast around corners; however, we do not simulate any of those things in Simutrans, so the speed limits need to be imposed automatically to simulate what sensible people running a railway would do in reality to ensure safe operation.

The imposition of speed limits by players is a different topic to the issue of differential speed limits which I believe is where this topic began. Differential speed limits, as I understand it, are used where different types of rolling stock have different characteristics which allow them to travel safely at a higher speed over certain stretches of line, such as the ability of tilting trains to take corners more quickly or the ability of trains with better brakes to stop in time for signals spaced more closely together.

In Simutrans-Extended these attributes are simulated, not by a fixed speed limit imposed on a section of track, but by a dynamically calculated speed limit for corners and dynamically calculated braking distances for signals. This gives the closest conformity to the modelling of the economic effect of various features of railway operation (such as signal spacing, curve radii, tilting mechanisms, braking capabilities and so forth) with the minimum of complexity both to code and to interact with for the player. The thing to remember is that the speed limit for a particular type of track is not intended to be a representation of the line speed limit of an actual piece of railway line: it is intended only to be a representation of the fastest line speed limit that that type of track will allow. The actual line speeds are calculated dynamically taking into account all relevant features.

Remember, the goal of Simutrans-Extended is to be an economic simulation, not a computerised model railway. Operational details that have minimal economic impact and which would be complicated to simulate (e.g. the fact that, in reality, speed limits are calculated by engineers when the line is constructed rather than calculated dynamically by each passing train) are generally not represented.