Well personally I would not allow to place and delete train anywhere on tracks. For this I still like the idea of depots. if sou want to purchase new train, if you want to delete it, or if you want to add a vagons, then Im totally fine with need to go to the depot. That is making route design more interesting in my opinion.
But if you want to just replace locomotive for a new one – let me do it instantly. Same for the trucks.