Are there programs running in the background? Internet and other games can take up excessive amounts of your computers RAM and cause your PC to go over it's limits. Trainz is forced to wait until it can get some more free CPU space so you can continue on. If there are no background programs up, and you continue to lag, it may be some engine or other asset you have currently on your route in Trainz. If this is not the problem, then your computer might be on the fritz. Most likely thats not it. Just need to get some more ram, or you could think about upgrading.
Steve