Another idea of mine is to enable stockpiling of resources which are used by the vehicles, of which the easiest to identify would be petrol (oil). Rather than having a max vehicles in game.
In a new game, along with the initial start loan a player gets an allowance of x of primary resources, if we make petrol (oil) the starting one for this explanation, a new player gets 10000 litres of oil and storage to house it, of which each vehicle takes an appropriate draw on per year, so a bus takes 120 litres a year of oil to run. The stockpiled resources would be housed in a relevant building, like a depot (for which a new sprite would need to be made), and each town gets a depot with a capacity of 10000 to start, as the town grows the capacity of the depot increases and thus more vehicles can be maintained.
When the out-going allowance of oil exceeds the incoming by transit, then the fleet is slowed down proportionately by the percentage difference of the above, and no more vehicles can be purchased, until such time that the incoming allowance of oil exceeds the outgoing. Slower vehicles would draw less oil, and thus allow some players to have considerably more vehicles than others at the expense of a slower return.
This would allow companies to operate with the amount of oil they can hold in their depot, and with big vehicles (like long trains over 7 tiles) limit how many vehicles they can maintain. So in other words it's a limiter on how much oil you can sustain, but unlike the limiting of individual genre's it's merely a different way of limiting.
Oil delivered to a town would yield considerably less revenue than delivered to an oil refinery (to make goods), but oil delivered to a town would make both the fleet run at normal speed, and dictate how many vehicles you could maintain.
