does anybody know how to clean brass cheaply and easily?
Plain old vinegar, but you could use some salt with it. Make a paste, maybe, and rub away. It will tarnish in time, so maybe you should go not-so-cheap and use MAAS. It will leave a coating that will still transmit current, but inhibit oxidation as well. Or, so I am led to believe. I’m sure you could clean yards of brass with one small tin/bottle.
Are you talking about track or locomotives? The above is fine for track. For engines, First you need to strip all the remaining laquer coat. Remove any plastic parts and bathe in laquer thinner or acetone. That ought to do it. Then, use one of the commercial tarnish removers with a Q-Tip to get any and all tarnish off. When you have it all nice and shiny, wash, dry, and spray with clear laquer.
For small areas, you can get a good result with one of those fibreglass pencils. Just beware of splinters, as they’re very painful and almost impossible to see. Hope this is of help!