(at least half of which would be very detailed research about why such differential speed limits exist and the economics behind them).
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.
