Ukaflyers response is a good one, basically you got lucky with the 300.
Here are some basics for Flyer steamers. The sum of the DC resistance of the armature and the field of a Gilbert universal motor is about 2.6 ohms. So if 8V is applied at startup the motor draws 3A for a short period as the speed builds up. The motor has a 6 to 8 Ohm AC impedance in operation pulling a short train, less up grades or pulling a longer train. The apparent DC resistance if a DC supply is used is about the same. The reverse unit coil, if early production has 15 Ohm resistance, if later production it is 27 Ohms. So at 8V it draws another 1/4 to 1/2A. A boiler smoke unit draws about 1/2A, a tender unit much more because it also has a second motor. Add to this the current to supply any illuminated cars in the train.
No HO Power Pack will reliably provide this much current. The VA rating is at rated voltage, typically 12VDC. A 35VA transformer will supply a maximum of 3A if the rating is at the output, much less if that is an input rating, more like 2A.
Gilbert made a lot of DC motored Northerns and 0-8-0's. The Permanent Magnet DC motors used a different armature that has a resistance of 2.6 Ohms to make up for the resistance of the missing wound field. No reverse unit was needed, these were efficient performing engines when new. Over 70 years the magnetic field strength of the AlNiCo field magnets will have degraded so they generally do not run as well as when new.
Try what Adriatic says, it is very inexpensive. Personally I still run mine on AC.
