I don't normally post here but I see you need some help. A Marx 1666 is usually easy to fix. Keep in mind you can probably buy a replacement 1666 for around $40 on eBay, so you don't want to spend a lot of time and effort on it.
I'd say about half of the old Marx locos I've dealt with just had dirty wheels and cleaning those wheels perked them right up. I don't know what it is about Marx drivers as opposed to anyone else's. But taking a cotton swab with rubbing alcohol to the four large driving wheels will almost certainly help. Swab the wheels, and once the swab turns completely black, repeat with a new, clean swab. You may end up using 8-10 swabs per wheel but that's OK. Also swab down the copper pickup slider in the center of the locomotive's underside. Sometimes I'll take a little metal polish to the raised portions that come in contact with the track, depending on how big of a hurry I'm in.
If you put the locomotive on the track and all it does is buzz a lot, the sequencer might be messed up. Tapping on the top of the locomotive behind the smokestack can help free the sequencer up. If that isn't enough, a few blasts of contact cleaner (available at auto parts and hardware stores, a can costs about $6) may help. Flip the locomotive over, and the sequencer is hard to see, but it's an electrical device near the front driver wheels. Spray some contact cleaner into there, then set the locomotive down and let whatever crud wants to flow out flow out. Let it sit for at least 30 minutes before you put it back on the track because you want all that contact cleaner to either flow out or evaporate before exposing the loco to electricity.
The loco probably also needs some lubrication. Automotive grease is fine; 3-in-1 or another household oil is adequate for oil but not ideal. I like Labelle 107 oil, available in hobby shops. Apply a single drop of oil to each axle, including the motor armature. Apply a bit of grease to each gear.
Marx trains were extremely simple and reliable. I think I've only ever found one that I couldn't fix by doing these two things to them.