To determine a connecting part installing position improve durability, and reduce manual labor.

In an electropneumatically connecting device for an on-rail trailer of a system of composing a train by connecting both connecting parts so as to coincide with each other by mutual guidance of a guide to bring a trailer 1 having a trailer side guide 20 and a connecting part 8 and a railroad bogie 4 having a railroad bogie side guide 18 and a connecting part 8 nearer to each other by a longitudinal movement, one 51 of an electropneumatically connecting device 50 being put in a connecting condition by butting them agaist each other is arranged in the railroad bogie side guide, and the other 52 is arranged in the trailer side guide 20. One of the electropneumatically connecting device 50 is supported in the railroad bogie side guide through a swinging mechanism, and the other on the trailer side is fixed and installed. A guide pin is protrusively arranged in one of the electropneumatically connecting device 50, and a guide receiver is arranged in the other. The swinging mechanism supports the electropneumatically connecting device 50 by an elastic member arranged between a guide inner wall on the railroad bogie side and the electropneumatically connecting device 50. A cover member and an operation member to open and close this are arranged on the trailer side.