Actually, just open the train.dat file in notepad, scroll down to where it says #CAR
you will see something like this:
#CAR
30 ; MotorCarMass
8 ; NumberOfMotorCars
30 ; TrailerCarMass
0 ; NumberOfTrailerCars
18.6 ; LengthOfACar
1 ; FrontCarIsAMotorCar
2.4 ; WidthOfACar
3.2 ; HeightOfACar
1.5 ; CenterOfGravityHeight
5 ; ExposedFrontalArea
1.6 ; UnexposedFrontalArea
Just change the number on where it says "NumberOfMotorcars" to the amount of cars you want. Then just edit the extensions.cfg to match the number of cars specified in the train.dat file.