Here's a short video showcasing a gameplay for LOCO Railroad 

What would You like to see in railway model simulator?

• Track editor [to create own layouts]
• Different modes [other than just control trains], for example ability to move goods between different industries [without economic factor]
• Changing seasons, weather, day/night cycle
• Realistic railway equipment: switches, signals

1. I'd say weather and nature, because scenery is the best thing about simulators by giving the feeling of exploring the world. You mainly need more complex terrain with mountains, hills, bridges, forests, river deltas, canyons, et cetera. Start with randomly generated hills and then erode using a vector-field water simulation to carve out realistic rivers in a large scale world. Then place buildings at strategic locations.

2. Regarding the technical part, I'm mostly interested in having a big variety of vehicle models and seeing people traveling to places. A simple thing like selecting colors and logos for the train makes a big difference in the feeling of customization. Being able to go first person and take a ride with other passengers like in "Mafia" would be a plus.