what threadlock is on MTH bolts for driverods

The wheels are metal, no issue there.  I've had a few that were quite snug from the factory, but I don't believe they actually use a product like Loctite (unless they began doing so after 2007.)  It's a fine-thread hex head machine screw.  If you have problems with a screw backing out, I would use the lightest-duty variation, which I think is blue- easily removed with hand tools.  My $.02.

I believe it goes purple, blue, red, weakest to strongest.

I hava a bottle of purple in my shop.

Right from Loctite-

Consumer grades:  http://www.loctiteproducts.com/threadlockers.shtml

Industrial grades:  http://na.henkel-adhesives.com...maryFacet=0000000389

You'll have to do some digging for the industrial side of their product line, rather extensive to say the least.

The Premier K-4 locos in my roster have blue Loctite on the main rod threads. Most of the time the insert comes out of the drive wheel, but the screw stays in the insert. It took heat on the screw heads to get them to come out. When replacing the screws, I use clear fingernail polish. It holds well, but can still be removed.

I use the purple thread locker for drive rods.  I've gotten a few for repairs that had the screws back out with predictable results.  I personally had my T1 Duplex back out a screw and pole-vaulted off the track when the rod jacked it off the track.  I was fortunate that I was running on the straight module and it just flopped next to the track, and not down to the concrete floor three and a half feet down!

Hmmm i didnt know that locktite has purple. (I thought blue was the weakest option) 

As for MTH models they use a thin clear liquid of the color blue. Over the years of bad or stretched traction tires, they do use something blue at the factory. When backing the bolts out for the first time you would hear a snap or click where the mysterious blue threadlock unlocks.