The set should be an AA set of #62 B&O diesels and a boxcar, hopper, tank car and caboose.
The original set transformer was probably the very small 25W #309 with output of 6-12 VAC. However, Marx will run just about any available transformer with a variable output. Ideally, it should be variable in the range of approximately 6-15 VAC (or DC) and have an output of at least 2A. Note that the powered unit has a two position reverse (forward and reverse only). Do not forget to clean and lubricate the motor, if needed.