CIM2 have a much nicer solution for this. The vehicles visit the depot for every revolution of the line then its cleand and repaired. If its not finished cleaning when its put out in service, the coustmers will not like it, if its not finished repairing the runingcost increase.
Also, one more feature i like in CIM2 is that its the depot that own the vehicles, then the lines rent vehicles from the depot with a given interval. In that way they don´t clump up. Also, there is no runingcost of the Vehicle when its in the depot, but there is a cost of the depot, the larger depot can clean a larger number of vehicles. If i remember correctly, the large depot can hold 30 vehicles and clean 3 simultaneous. You can still have like 50 or so vehicles attached to one depot, but they can´t be in the depot simultaneous (they have to wait on the road outside until there is a free slot).
If needing to replace vehicles just simply go to the depot and buy some new ones and sell of some old ones.
It might not be perfectly adaptable to train fever, it is probably a good model to use.