I have found some excellent websites that are very useful for calculating the power/tractive effort of steam locomotives from known technical data. Although the tractive effort is relevant only to Experimental, I thought that this would be of sufficiently general application to post here.
Firstly, this (http://www.smex.net.au/Reference/TractiveEffort02.php) web page has an automatic calculator for tractive effort and power (albeit one has to put in a "loss factor" which is somewhat of a guess) for non-compound steam locomotives (most steam locomotives are non-compound). this (http://www.tt-forums.net/viewtopic.php?f=33&t=31980) post on the Transport Tycoon forum, meanwhile, gives the calculation applicable to compound locomotives (and contains the link to the first web page).
This (http://orion.math.iastate.edu/jdhsmith/term/slindex.htm) website, meanwhile, provides much of the data needed for the tractive effort/power calculations in the calculator above for a huge range of steam locomotives from all over the world, including a large section on British locomotives.
Edit: Testing shows that, while the tractive effort seems sensible, the power calculated is much too high, and should be reduced by as much as half. However, it might be that it is necessary to use a reduced boiler pressure when calculating power; whilst it makes sense to use the maximum working boiler pressure to calculate starting tractive effort, as the locomotives will generally have a well-pressured boiler on starting, this does not apply to continuous running. Setting a boiler pressure of half that indicated seems to produce figures that are similar to those calculated from actual performance data, but further testing is needed.
Edit: Update the link for the tractive effort calculator
The power this calculates is correct; it is the maximum power at low speed. As soon as the cylinder consumes steam faster than it can be delivered (about 20 km/h or so) the power will drop. To calculate that speed, you need to know the maximum evaporation area and the drop along the lines to the cylinder.
The main advantage of electric or diesel were the constant power over the full range of speeds ...
Hmm, are you sure that that is the intention of the calculator (to show maximum power at low speed)? It specifically gives speed as a parameter, and shows higher power at higher speeds, and doesn't show the speed above which the power falls off.
I do recall reading or watching that the steam pressure needs to be built up before maximum power could be delivered. And once it has been delivered, the locomotive would need to slow down to build up pressure again.
The power drop off was more in relation to the amount of time it took to consume more steam than the boiler could produce.
Resurrecting a somewhat dead topic in order to have a repisotiry of relevant information, this (http://www.germansteam.co.uk/Traintiming/Recent/timingdata.html) website gives interesting information on German steam locomotive timing data, which might be of use.
A little more necroposting is in order, I think, but these are some useful websites.
Train Testing (http://www.traintesting.com/) gives some useful performance logs of both steam and modern traction from the 1950s onwards; meanwhile the Loco performance (http://locoperformance.tripod.com/) website gives a range of 1950s and onwards data, including data for the performance of preserved steam locomotives.
I hate to spoil the fun, but I tested the first link using the data for DSB litra S (http://www.denstoredanske.dk/@api/deki/files/78109/=DSB_Litra_S.jpg).
I added the numbers and it gave me 608 HP@98 km/h. However the test run in 1923 rated it at 1000 HP@98 km/h. It goes with the story that it turned out to be stronger than expected and it was strong enough to be used for freight even though that wasn't originally planned.
I tried another one and it ended up with 400 HP though I suspect 700 appears more like it when looking at how it performs.
I wouldn't trust that formula at all.
A little sidenote. You have probably seen DSB litra S without knowing it. It was used in 007 Octopussy where it pulled the circus train. Repainted to look German and recorded in England. Yep, that's movie realism for you :P
Ahh, yes, the horsepower calculation is a long way off, as I have since discovered. I have since recalibrated all of the steam locomotives based on in-game performance: the recalibrated locomotives will be in the next pakset release. It is accurate for tractive effort, however.
Welcome to the forum, incidentally!
More interesting articles on this topic:
http://www.tandfonline.com/doi/pdf/10.1080/00966665.1954.10467614 (http://www.tandfonline.com/doi/pdf/10.1080/00966665.1954.10467614)
http://www.rmweb.co.uk/community/index.php?/topic/3790-steam-loco-power-ratings/ (http://www.rmweb.co.uk/community/index.php?/topic/3790-steam-loco-power-ratings/)
http://www.thewoodheadsite.org.uk/Perform/Perform.htm (http://www.thewoodheadsite.org.uk/Perform/Perform.htm)
http://www.steamindex.com/magrack/locoperformbul.htm (http://www.steamindex.com/magrack/locoperformbul.htm)
http://5at.co.uk/index.php/references/simulation-software/software.html (http://5at.co.uk/index.php/references/simulation-software/software.html)
http://www.martynbane.co.uk/modernsteam/pg/linda/ngrs.htm (http://www.martynbane.co.uk/modernsteam/pg/linda/ngrs.htm)
http://www.steam-loco-design.co.uk/zb_article_4.html (http://www.steam-loco-design.co.uk/zb_article_4.html)
http://www.steam-locomotives.co.uk/Articles.htm (http://www.steam-locomotives.co.uk/Articles.htm)
http://www.lner.info/locos/C/c1.php
http://www.lner.info/locos/D/d14d15d16.php
An old (1900s) book on the subject is available secondhand here:
http://www.bufferbooks.com/page11.htm (http://www.bufferbooks.com/page11.htm)
Reviving this old but useful thread, which acts as a repository of useful calibrating information for steam locomotives, this (http://www.steamlocomotive.com/locobase.php) website contains a wealth of physics data (firegrate area, tractive effort, weight, etc.) of steam locomotives from around the world, and I have been able to use it to fill in some missing details in respect of some locomotives and adjust their data as a result.