Depending on how old they are and what kind of lighting they have, they will probably be fine. If they are designed for lower voltage, though, the bulbs will eventually burn out. At that point, you can replace them with higher voltage bulbs, or with LEDs.
These lights can use a lot of power. With one or two cars, you probably won't notice it unless your DCC system is already underpowered for your layout. Each car, though, will take a bit more power, so long strings can affect performance.
One thing you might consider is a siding with a kill switch where you can put the cars and power them down when not in use. This will both reduce power consumption and increase bulb life.
It takes an iron man to play with a toy iron horse.
I have some older Bachman Passenger cars that light up.I notice when I place them on my new Dcc track they still light up.What effect if any will this cause.Good or bad for the cars or track.Thanks