I am having a very similar sparking problem and I’d like a clarification on what “spring” you are talking about. The truck pivots on a flange of the motor bracket. That’s where I see my sparks. Where is this spring ?
Most pilot trucks have a spring somewhere that pushes the truck down. This pressure keeps the light truck railed better than gravity alone. (is this the EXACT same engine?)
This spring appears to surround a long rivet's shaft(?) The rivet floats vs being held fast, the spring holds it up. A bad spring cant even hold the rivet up so it drops slightly, hitting the rail.
Your sparks sound like they are up high at a connection point. If it doesn't bother performance, I would suspect your sparks are minor, like that of a small carpet static spark. That could be the normal electrical flow of ground/common to the frame when the rubber tired drivers loose flange contact, or the trailing truck looses contact etc., forcing a bit more electricity to be drawn from the pilot truck. Dirty wheels or track could cause this too.
If that is the case; motor might have a slight bit of trouble pulling a good common/ground. You could tether an additional ground from the tender wheels/chassis to the engine chassis.
Also, pressure has a huge effect on electric connections. The more the better. So a spring at any connection, applying pressure to the connection, helps delivery of power. A weak trailing truck connection would cause a heavier draw thru the P-truck too, etc etc etc.
Without a photo or drawing, only folks with the loco or tons and tons of repair experience know for sure. Not that many many others aren't capable, but it is real time consuming haphazard guess work. A great repair tech may have never seen one before either, let alone worked on one.
Don't folks have cameras anymore? Repair threads seem notably void of photos lately in comarison to past years imo. I know some techs are plain tired of having to ask for numbers, photos, etc, and skip right over some posts, fyi y'all.