Why is it needed to replace bad condition vechicles

[barack ofortnite]

as far as I can tell there is no diffrence other than looks.

[mmmcheesywaffles]

They cost more to run and pollute more. Also they attract less customers.

[lost]

Originally posted by mmmcheesywaffles:

They cost more to run and pollute more. Also they attract less customers.

They do not cost more to run. Nor does it impact passenger numbers except due to the indirect knock on effects of increased pollution.

[Vimpster]

The answer is that you do not need to replace bad condition vehicles. You are not intended to either. The only reason the game gives a message about it is to remind you to temporarily increase the maintenance level if you care about pollution. Beyond that you can safely ignore any messages about old vehicles.

[Autocoach]

Don't forget that the messages can also be disabled

[genemead]

No, you don't have to replace them. You can just keep the same ole trains running forever. No problem. Feel better?

However, you're only assuming (guessing) it's OK not to replace trains cause that's "as far as you can tell".
There's people who have gone into this maintenance thing MUCH deeper than just "guessing" and can back up their findings with facts. (Search is wonderful. You should try it sometime!)
https://steamcommunity.com/app/1066780/discussions/0/1740009710701743742/

As it turns out, you're mostly right.
But if you don't mind old, rusty, greasy, stinky trains running around in your pristine environment or cities, then that's your choice.
(I couldn't stand it, personally.)

[lozacenz]

Originally posted by Windows Millennium Edition:

as far as I can tell there is no diffrence other than looks.

Vehicles that travel close to residential areas have a negative effect on city growth based on their emissions value. Being old and worn out makes this worse.
That's it.

There is no need to replace them at all, save that higher speed, higher capacity vehicles are generally more profitable (though there does come a point where there are routes that are too short for a given vehicle to actually benefit from, say, a higher top speed, so keeping old ones around on those routes is useful).

As for increasing maintenance, at it's highest level it reduces the vehicle's emissions back to the minimum that vehicle is capable of (and then keeps them there).