I have been using MSTS for over a year and a half, and find it to be worth the money. Unfortunately, I have not worked on my HO layout since I got MSTS. The two US routes that come with the game are OK, but they are not my favorites. I bought Sandpatch, the DH Bridge Line, and the Erie mining RR, and I made my own LIRR Port Jeff Branch. I bought Erie Mining for the RS11's. Learning how to make my own was the hardest learning I have done in many years, but it was worth it.
Train-Sim.com has so many add-ons. Learning how to take cabs and sounds from one loco, and adding them to others can be quite a challenge.
It would be nice if there were one product with LI routes and equipment, but there is not. Nevertheless, you can piece together enough to make a good time. I have LIRR RS1's, RS3's, C420's, GP38-2's, MP15's, and FA's. One RS-3 I painted, all the others were from train-sim or a guy in Quebec. I have a C-liner and an H16-44 that I would like to repaint to LIRR livery.
I have found the PRR signals available at Train-Sim to be too faint to read until 50 feet away. So I use BO color/position signals.
Around January I felt I was spending too much time on this, and I drifted to other hobbies, but I returned last week. Just for fun I piloted train 656 with a GP38-2 and 8 coaches east from Hicksville circa 1984. I had a moving meet with RF61 (2 C420's, 8 coal empties and 8 salt empties) west of Syosset. I was 7 minutes behind schedule when I decided to quit at Northport.
The bottom line is: you can do OK with a minimum of effort, but if you want lots of realism, it will take a lot of time to customize your sim.