Most of the older F9s that were in train sets were 4 wheel drive. If that is the case it has an inexpensive drive train.
You can look at Bachmann on line and see what type of drive you have.
If it is an older 4 wheel drive unit it has a drive truck that is usually not in stock. You can contact Bachmann and check on repairs. If it is a 4 wheel drive, I would suggest replacing it with a newer locomotive
1. Is this a older 4 wheel drive locomotive?
          A. Does one truck have two axles that turn.
          B. Are there rubber traction tires on the wheels of the other truck.
2. Is this a newer 8 wheel drive locomotive.
          A. Do axles on both trucks will not turn.
          B. There are no traction(rubber) tires on the one of the trucks

The other possible problem could be dirty contacts where the rail joins together. Reset all the rail connections and see it that helps.

Good Luck, John

FlyBoy,
Make sure the CRC is CRC2-26. Many of the CRC formulas will attack plastic and damage your track.
Regards, John

When we are planning for posterity, we ought to remember that virtue is not hereditary. <> Thomas Paine

I have swapped a 4-8-4 can motor chassis with an older pan cake chassis on two occasions. The first was from a Union Pacific Niagara style train set engine to a Santa Fe style 4-8-4. The second was from a spectrum NYC Niagara to a Santa Fe style 4-8-4. The chassis are the same, but the body, valve gear and the cross-heads must all be swapped. I suspect that the GS4 is the same chassis also.

Here is link that will help to identify a used can motor chassis.
http://www.tycoforums.com/tyco/forum/topic.asp?TOPIC_ID=11848&SearchTerms=Bachmann,4,8,4

NOTE: I have no experience with the current production 4-8-4 engines.

Regards, John

While we should never give up our principles, we must also realize that we cannot maintain our principles unless we survive. <> Henry A. Kissinger

Captain,
There are three main generations of Bachmann 4-8-4 engines.
1. The first generation was a pancake motor design > Cheap and ran poorly < Avoid these
2. The second generation was a split frame design with a can motor < Runs good no parts < many used engines around
3. Third generation current production > improved design more rugged < no first hand experience with this type

Here is a link that will help you understand the difference between the early engines and the second generation.
http://www.tycoforums.com/tyco/forum/topic.asp?TOPIC_ID=11848&SearchTerms=4,8,4

To get to the point, you can pick up an old second generation 4-8-4 and rob the parts to fix yours. The direct replacement engine is a Overland like yours or a NYC Niagara. Be careful as Bachmann used the Santa Fe Northern in early overland sets. The drivers for the second gen 4-8-4 engines are interchangeable, but each type needs the correct cylinders and valve gear. Look at the pictures and be sure the drive rods are in line, these engines have a bad habit of the center axle plastic breaking. Make sure you use plastic compatible oil on these engines!

I have noticed more vintage parts from Bachmann. It would be really great if they would provide the drive axle center inserts as spare parts. That appears to be your problem.

Good Luck John

A man may imagine things that are false, but he can only understand things that are true, for if the things be false, the apprehension of them is not understanding. <> Isaac Newton

Well it is almost 2 years since I posted the original post. The Bachmann line of engines has improved in many respects. they are a very good value in today's market. The parts and support have improved. The old standby Athearn has become the not in stock supplier for parts and engines.

These improvements have lead me to decide to use the Bachmann F-unit drives in my current roster of engines. I will start to replace my current Athearn F-Unit drives with the newer 8-wheel Bachmann drives. My newer engines will be Bachmanns. I have a number of EMC FTs and plan to purchase ALCO S1 and S2 diesels that will fit my WWII era well.




Regards, John **********************

You can always count on Americans to do the right thing - after they've tried everything else. <> Sir Winston Churchill

Jack,

1, You can manually reverse the loop with a DPDT switch.

2. You could use 1 light sensor to trip a reversing relay if you only enter the loop in the one direction.

Regards, John

A man oughta do what he thinks is right. <> Hondo Lane - Hondo

If you need a diagram, you might try this web site.

http://www.hoseeker.net/lit.html

Regards John

Foreign aid might be defined as a transfer of money from poor people in rich countries to rich  people in poor countries. <> Douglas Case,

Au Contraire,

Athearn was doing  this very thing for years in their Blue Box, and RTR line. They used the same motor/flywheel and four wheel  trucks for all for all 8 axle diesels. You could pull a truck off of a U-30B and change the side-frames and put that same truck on an F7.

If you look at the Bachmann standard line 4 axle diesel trucks, you will find they appear to be interchangeable with a side-frame swap. If Bachmann will standardize on a motor and part numbers then the hobby will have a source for parts and kit bashing. Bachmann will have a better way to build and stock parts.

Regards, John *****************************

What was the best thing before sliced bread?   <> George Carlin

Follow the link:

http://estore.bachmanntrains.com/index.php?main_page=product_info&cPath=66_68_91&products_id=4398



Regards John *****************************

I never drink water; that is the stuff that rusts pipes. <> W. C. Fields

Dude,
Bachmann in last few years has introduced in HO:

A new 4-4-0 American. DCC and Analog
A new 4-6-0 Ten Wheeler DCC and Analog
And a new DCC Sound and Analog 2-6-0.

not to mention Their old reliable 0-6-0, 2-6-0, 2-6-2 & 2-8-0 are still abundant

John

A fine is a tax for doing wrong. A tax is a fine for doing well. <> Unknown

How in the world will a cracked gear cause the loco to run wide open in one direction? You have a DCC decoder problem, a wiring problem, or a programming problem. If this is the only engine that has this problem.
1. Then Reset the decoder.
2. Check the wiring harness for shorts or damaged insulation.
3. If you can then purge the DCC command station of all stored Addresses
4. Remove the decoder and replace with a known decoder.

What type of command station do you have?

Regards John ****************

A couple more generations and we will have the knob holders bred out of the gene pool...); <> Benny

For the most part the answer is no.

Most modern manufactures do not stock parts for out of production models. If they do, then only for a short time. The current production model may be revised and the older parts may no longer be stocked. This is the way it is now that overseas production is the normal chain of supply. I keep my old Bachmann 4-8-4's running by buying parts and old engines off of EBay. I have learned to identify models that use the same parts as my models.

Bachmann does have a lifetime policy to repair or replace models that were purchased new. You may not always get the same model back, but they should get an A for trying.

Regards, John *************

Scientists dream about doing great things. Engineers do them.  <> James A. Michener

Young Man,
You are going to be amazed at just how smart your father gets by the time you are 18.
Regards,
John
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Comfort and prosperity have never enriched the world as much as adversity has.  <> Billy Graham

Are the wheels slipping, or is the motor running and the wheels not moving. Some time a gear may be cracked or spinning in the drive system. A binding drive train may also drain power and keep the engine from pulling well
Good luck
John

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I have never been lost, but I will admit to being confused for several weeks.  >> Daniel Boone

844,
Bachmann makes a whole line of EZ track turnouts both #5 and #6, Along with many curves that are greater than 22 inches.  The track combinations are pretty limitless with their track products.

http://www.bachmanntrains.com/home-usa/products.php?act=viewCat&catId=195

By using plywood or cork under the track you can combine other brands of track for yards or sidings. You may have to do surgery with a razor saw from time to time to get the track like you want it, but such is life.
Regards,
John
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Information is not knowledge. >> Albert Einstein