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.
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
post on the Transport Tycoon forum, meanwhile, gives the calculation applicable to compound locomotives (and contains the link to the first web page).This
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