From UKTrainsim forums this post describes how to set it up in Steam so that you don't have to click the box everytime:
Re: Update?
Postby Griphos on Thu Oct 13, 2011 9:29 am
No, you set it in Steam. Go to the games list. Right click on your TrainSim 2012, and click on Properties, and then the button called launch options, and then type in the line above "-FPSLimit=25" or whatever you want. I set mine for 30, and it works great, but I put in the launch options so I don't have to remember to click the box in settings everytime I start.
http://forums.uktrainsim.com/viewtopic. ... a&start=60