my n scale train slows down after about 5 minutes.

[hobo Walt]

First let me say I'm a complete newbie. I like trains want to start a new hobby so I bought a small set. It's Bachmann N scale from 1974 new old stock. It came with a very small oval track. About 18" across or so. I believe it's a Santa Fe Engine. I took it out of box connected it all and started it up. It pulls the four cars it came with, but that doesn't appear to be the problem. By itself, if I let it run for about five minutes or so it will start running noticeably slower. I count "1 thousand, 2 thousand" etc. It appears to run the circle in about 5 seconds very consistently for about 5 minutes or so and then over the next couple of minutes it slows to about 7 seconds.
Also, if it's relative, In the first few minutes of running the train there was an occasional high pitched whir sound accompanied with the engine appearing to stall for just a split second. This did it a few times and then settled down and hasn't done again in since.
I can't think of what else to add. Oh, I looked forward to getting sucked into this hobby. But I do want to get this figured out before I really dive in. I bought this on Ebay and want to know if I should work on sending this back and start again or if this is something simple. Thank you to all the more experienced folks in advance.

[ABC]

Try cleaning the track, wheels, motor, and connections. And then lightly lubricate the track and engine with conductive lube and the gears and motor with plastic compatible lubricant like Labelle.

[brokemoto]

What Santa Fe engine is it?

I am suspecting that it is what B-mann sold as an F-9, but I can not be sure.  Those things have quirky operating characteristics.  There has been much improvement to N scale motive power in the past thirty-six years.  Much of the equipment manufactured in the 1970s is JUNQUE by to-day's standards.

If you want a similar and inexpensive locomotive from B-mann, look for a SPECTRUM F-7.  I am not  a major fan of them, but for a beginner, they are allright.  They will run more dependably and better than that old locomotive and the scale and appearance is much better, as well.  Be sure that you buy the SPECTRUM F-7.  B-mann also sold a PLUS F-7, and the shells for the PLUS and SPECTRUM are interchangeable, but there were problems with the gears on the PLUS.  B-mann addressed the problems and upgraded the thing to SPECTRUM.   Many hobby vendors stock these things and they are widely available at shows and on FeePay for good prices.  Check modeltrainstuff.com or trainworld.com for the best prices.  I have dealt with both firms with good results.

To answer your question, I am guessing that there is carbon caked on the armature and possibly in the insulating gaps.  A serviette soaked in Life Like track cleaner will help.  If this is an open frame motor, simply wipe the armature with it where the brushes contact it.  Put the chassis onto the track, run it.  It will sputter and stumble at first, then take off flying.  Repeat the washing a couple of times.  If that does not improve things, you have another problem. 

Your better bet is to buy a better locomotive.  B-mann and many other manufacturers sell good products that meet to-day's standards.