Do you mean that you don't have the original power supply? Or do you mean that you don't have the "load" (the second carriage)?
It is helpful to see the original power supply. To see the size of the transformer, the gauge of the wiring, the ratings of the other components to help make an educated guess about the current (assuming the voltage can be calculated from the transformer specs, etc.)
If you don't have the original power supply, then it is customary to arrange for a temporary power supply: a "bench supply", or in your case, maybe a big pile of heavy batteries of the proper voltage output. Then you can place an amp-meter in series to actually measure the load.
Or you may be able to calculate the load. For example, if it is all the lighting in the carriage, you can calculate each lamp, and multiply by the number of lamps, etc.
But if the load is some big solenoid or relay, or the brakes, or something, then you don't have much choice but to actually measure the load directly using a temporary supply.
Or, you could simply over-specify the replacement. 50A or even 100A diodes are not all that outrageous.
Remember, however, that those old selenium rectifiers were nowhere near as efficient as modern silicon diodes, so you may get a higher output voltage than the original.
We sometimes must insert heavy series resistors to "simulate" the voltage drop across the old selenium rectifiers.