Simply 'middle carriage' (British English) or 'middle car' (American English).
However, if it is only a 1 carriage train, then the middle carriage (and indeed, the only carriage) is just called the carriage, car, locomotive, or unit. Two-carriage trains do not have a middle carriage.
On trains with more than 3, and an even number, of carriages, the middle two are called the 'middle carriages' (or sometimes, more rarely, the whole train except the two end units). On trains with more than 3, and an odd number, of carriages, the middle one carriage is called the 'middle carriage' (or, more rarely, the middle three, of even the entire train except the two end units).
A passenger train
Train Passengers.
The engineer runs the train.
Chew chew train.
A train that carries goods is called a freight train.
Each part of a train is called a car.
a mono rail train
In the middle of night the train reminds the poet about the huge elephants moving in the dark night..Hope it helps!
Page 8, on the middle gate, middle half, wire train, turn book sideways
A compartment
Rumble!
The train bearer