Some of the Train Sim stuff is massive in terms of file size - most recent route releases can approach 1Gb once unpacked. I think there is a way within Steam to deselect the add-ons you don't want and Steam will remove some of the files, but bear in mind if you have any freeware routes that reference payware stuff these may no longer work or display correctly.
If you're into train (and flight) sims as I am, with MSTS and Trainz also installed along with loads of additional content you need plenty of HD space!