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Here's a classic from 2003: Rail3D
Its not as much as a train simulator as it is a railway simulator. You create tracks by clicking and dragging (and it bends to whatever shape you wants), at each "node" you can add features like stops, reversing, signals, and objects can be set to clip to track.
It cannot have above 20 fps (its a software problem, not hardware), but still makes me lose hours and hours creating and modifying railways.