Fyi The 6w & 20w posts are the constant voltage (v) 6v & 20v, not watts. Watts is a voltage/amperage combo formula you will seldom see referd to in trains outside of transformers.
And these posts are long because you seem quite new and Im trying to cover the bases widely.....or becuase I always post long 
Comments on my comments, to clear that up more.
Lightly crimping the adjoining rail over pin notches will improve connections. But clean connections are important. If the track is rusty try removing pins and dunking in "Evaporust" (common) rinsing and drying well...just remember, new track works like a million bucks.
Look at your track pins too. Some O27 got hollow pins rolled from sheet metal. They tend to rot inside out, and can get red hot with just the current of the loco drawing across them. I suggest buying solid pins for the center rails. Outer rails too eventually, but they have twice the metal so carry less current per pin and arent as risky there. Feeling CAREFULLY for heat is another test for shorts and resistance too.
Like Moonman said; more power feeds. 20ft of track needs more connections than one, especially old track. You need to run whats called a "bus" circuit. A fat wire to feed lots of power a long way, and smaller "drops" at each connection point. Id run at least four drops on an oval that long, more likely 6-8 & right at the curves is best, one for each leg of a turnout or X track crossings are a good idea too.
I cant find an engine reference in Greenburgs or online for 7477. Its not an 8377 is it? Military? That is dc only, so it likely has a toasted motor now. A dc motor can run a few seconds on ac, but will burn up. Ac motors can run on dc volts mostly, there are some exceptions, but no damage occurs.
It is cheap to replace those dc motors, but it will need a bridge rectifier(-$10) added inside to use ac and thats one direction only, or a switch to reverse, or an electronic e-unit (reversing, board) which might have a bridge rectifier built into it; check. (Always save an old motor for the gear, just in case. Get a box for "stuff" going. Tackle box?)
Even if that is the wrong engine # Im guessing with, see if you can tell, does the motor look like a toys motor ?(can motor, still good, they got better too) or does it look like a small fan motor?(Pulmor ac/dc open frame type).
If its a can motor from that era, its likely dc only, likely cooked. You would need to check for an electronics board possibly too. If you have hands on abilitly, the knowledge you need is here for the asking. As simply put as you need it, or as in depth as you want it.
A pulmor can be serviced. At home, with folks here guiding, by reading old threads, watching on youtube, etc.,or by a pro if you like. Its not hard
The id has got me going.....Somewhere on most engines is a "roadname" and a number, list that here. If your giving us a set number, thats of no fast help, but another ID oriented site might like box or train photos if its rare.
Note how many wheel sets are on each truck. 4, 6, or a combo?
And look at these body styles too- Carbody/ A unit (chuggington) , gas/diesel/electdic switchers("critters" by size) are small and squared gas/diesel, NWs or SWs are the bigger common "switcher", RS is an industrial road switcher(does both, med-long, rounded edges design) , and a GP is a med. industrial hauler, a U boat - (longer?) industrial, FM Train Master heavy industrial, very long and doesn't like 027.
Carbody A units(with cab) are E (3 axle per) & F (2axle per) , the B has no cab, adds power. An Alco A unit has a similar look, less curve on the nose top, 2 axle per. truck, there a Bs for those too. An Alco is not a carbody its built on a frame. A carbody's frame is integrated with the body. I.e. The body is the frame. Lionel Es and Fs are usually jewels, the Alco tend to be more toyish.
