Dovetail Games is currently running a flash sale on a a few of the add-ons for Train Simulator. These include different things like new trains and scenery.
There are ten different bundles to choose from with the cheapest being $28.46. The most expensive of the batch is $39.56. You can go ahead and check the Steam listings to see which of one appeals to you the most.
The flash sale ends on June 1st, 2017. FSX Steam Edition also has a flash rale running at the same time.