Knowing what truck type we are talking about would dictate 100% "proper" maintenance.
WD-40 is wire dry formula #40, and should be avoided unless you want to use it often enough it never fully dries.
It is not a "real lube". Its a temporary band aid at best.
You make have also washed away debris, or just moved it around.
It might make a good track cleaner
But it would likely rust it as well without a touch of ...oil.
Find a plastic safe lube and you'll be fine if you use common sense.
(But Delrin is a self lubricating plastic. Polish the axle tip, and wipe it with oil rag to stop rust)
On metal, motor oil will work fine and dandy.
Oil will collect dust. Heavy oil will collect dust faster, and you'll end up with more gunk, with each extra bit.(common sense
).
Almost bathing trains in oil, my grandfather was a Lionel collector that did fine with oiled wheels for 50 years ..oiled, wiped, and let to sit on a towel overnight, re-wiped then ran. He likened it to cleaning his Garrand, and I'm sure the owners today have fine specimens.
I personally like motor brushes, and that area, to be dry.
Another here on this forum, also an electrical tech in his line of work, swears by coating the whole shebang in motor oil.
My point with the various opinions is, folks have varying opinions on oil, and how to use it, but you'll be hard pressed to find any that swear by WD-40 long term.
Oil-
To much = messes, fouling of electrical(brush dust), and even loss of traction.
A tiny bit = a protective coating from oxidation as well as lube.
No oil = oxidation, and excessive wear.
Choose your poison.
