To use a capacitor, it would need to be before the momentary switch that fires the solenoid as it would need to charge from a constant supply before discharging. If a large enough capacitor was installed after the momentary switch, it would actually take energy away from the solenoid (as it charges) if the momentary was held for a short period of time. The idea of using a capacitor is a possible solution though, as a sufficiently large capacitor placed across the leads of the power pack output would help reduce the voltage drop (momentarily) due to the sudden current draw. Before I would try to make modifications to allow the small transformer to act more powerful, I would be inclined to get a more powerful source. You could build one that would supply 2 amps for under $20 with parts from Radio Shack. If you do want to try a capacitor, get one rated for more than the output of the power pack (50 volts DC would suffice) and get the biggest monster you can (something in the 3000uF range or higher). Anything smaller would not store enough energy to make a difference. Better yet, stick a few of them in parallel for higher energy output. But, as stated before, you are approaching the expense level of building a better power source...
I have attached a schematic for a simple supply which should be able to fire any switch machine with a reasonable cable length (under 100 feet). I have a couple of these located around the layout that I use to power switch machines using relays from a master control board. Feel free to ask for clarification if you don't understand the schematic.
Tom F