I don´t get why there would be a force replace vehicle at all. Espcialy with some of the trams that are old before they are out of date. There should be a refurbishment option where you just zero the age of the trains, trams and busses for a give amount of money.
Also, especial for trains, some locomotives that get out of date don´t get a good replacement, for example the 4/4 is a great general purpose light locomotive that is relatively cheep to run. But the only somewhat simular replacment its the nohab that is a diesel and a lot heavier