Dynamic weather is indeed a very good idea. I always miss them from city-building games (also, day and night cycle can be very spectacular and make the game more immersive – too bad it wouldn’t really fit into Train Fever).
Those points Scandi recommended are good, however I fear they might be hard to implement. But dynamic weather should be incorporated in the game! Also, if there is snow, well, it would be awesome if it could make certain sections impassable, so snow plow trains would be necessary.
But first of all: give us dynamic weather!