Sounds like a warranty issue

The myth persists that all old toy trains are worth more than their weight in gold.

Yeh.  Now That it's on the internet all the taggers and scrap vultures will be showing up in droves.

Broken record:

Used model railroad locomotives are the ultimate "pig in a poke."

https://en.m.wikipedia.org/wiki/Pig_in_a_poke

Verily, verily, I say unto thee. Bewarest thou of the used model railroad locomotive and train.  Knowest thou not of the indignities and sufferings it has suffered, be it having been run to the ground, or used as a weapon against a younger sibling. Knowest thou also not of any hidden terminal internal injuries that may. exist.  Spendeth thee wisely thy earthly treasure, on known quality, therefore avoiding weeping, wailing,  and gnashing of teeth.

I've seen people trim the old style couplers, commonly called horn Hook couplers, down to the bare minimum, removing all of the bottom trip pin and the horizontal piece to the side of the knuckle and get them to where they will mate manually with knuckle style couplers that are fairly standard across Model Railroading today.  They will still spring to one side, but you could put a piece of a toothpick in the pocket to stop that. 

It's not the ideal solution, But it's free because you already have the couplers and you don't have to worry about trying to install new items on the cars that may not readily accept them.

Change the railroad to DCC.  It's a big jump if a number of DC locomotives have already been purchased.  But if not, if you want the advantage of DCC in certain accessories only, I don't think it's really possible to do what you want right now.  We do have some electrical gurus on this forum who can probably answer this question better for you.

Unbelievable how much is preserved there. Not in the best condition, but it looks very dry there which means little deterioration.  I really like the Baldwin 2-8-2's, They appear to basically be a catalog design meant for short lines and logging trains common in the early 1920s, there are multiple similar American examples.  I wonder what the track gauge is there, it looks very narrow.  Also incredible is the complete lack of gang type tagging graffiti, imagine what that stuff would look like if it was in the US somewhere.

Unfortunately, all it would take would be one wrong-minded Political revolution to swing things the wrong way for this collection and turn it into scrap. I wish that someone could get those American engines out of there and into the museum that they really belong in.  There certainly are numbers of people who would view all of this as representational of outside foreign big business and political intervention, as well as destructive of environments and native cultures.

I'm old school too, Been modeling since the 1950s. But times change.  You're probably going to need by another model. And instead of buying another broken one to get the part, I'd buy one that runs and keep the broken one you already have as a parts reservoir.

if you think times are tough now, look back to the 1940s and 1950s when you weren't a real Model Railroader unless you scratchbuilt a steam locomotive out of sheet brass.  Some guys back then wound their own motors. Back then, you pretty much needed a machine shop to be a Model Railroader. When Varney, Mantua, and Model Die Casting locomotive kits appeared, it was a shower of blessings from on high for modelers.

That I don't know, having a different systems.

Start with a local hobby shop that sells Trains, or with a local Model Railroad club, if either of these exist in your vicinity.

You can also almost be certain that the train will not run if it is sat idle all these years.

The Appraisal value of any used HO set, any brand, any scale, any age is close to zero.  The untrue myth persists that any old model train is worth its weight in gold.

There is not a similar chart provided for a N scale ST 40 but the one for the N scale SD 45 looks almost identical to the one I posted the link to.

I have some more CV settings for you to try but let's get the speed issue fixed first.

I checked and it looks like you have the sound value decoder.

Before you do anything, try running that engine on another layout with a different system to make sure it's not your power unit.  Also try cleaning the track and see if that helps.

There's limited motor control CVS In the sound value decoder compared to more advanced ones.  But try advancing CV  2 up one step at a time And see if that doesn't give it a little more kick.

You can also try doctoring CV 5 and 6, "v" meaning "velocity." 

CVs 3 and 4 can be a lot better.  Try values of 50 and 35 respectively.

The below will prove useful over time.   Please note that  CV defaults aren't necessarily the best setting.  And you can always return any CV to the default if you're unhappy with what you set it at.

https://soundtraxx.com/content/Reference/Factory-Installed/Bachmann/SoundValue/bachmann_ho_gp40_sv.pdf

I

That can all be improved with motor control CV's.