Looking for super hard substance/filler

[Komrade]

I have rolling shelves on wheels/rails.
The rails may not have been installed properly (as they were re-purposed), and I am hoping to avoid trying to remove the shelves and try to re-install the rails.
The wheels hit a gap in the rails and the shelf gets stuck.
I am looking for some filler that will harden enough to withstand hundreds of pounds of pressure per inch without crumbling.
I'll take any other ideas. Pictures to follow.

 

[Komrade]

First Picture reflects what needs to be fixed on the gap between rails.
Second picture reflects material that really should have been used and likely "improperly" installed rail

 

[oosik]

OK - it is not the "gap" that is the problem. It's that the two sections of rail are not aligned. So - top picture - take out the two purple trumpet screws on the right side - align the rails - use the two vacant holes in that rail to install new screws to hold everything in place.

 

[Komrade]

I should have elaborated on where the gap is. It's not between the rails (and I think they're sufficiently aligned).
The gap is in lack of steel reinforcement in aluminum rails at the ends (if you look closely, you'll see about an inch of black filler missing). wh
Once the little shelf wheel jumps off the still filler, it hits about 1/16 of an inch of a hole, which is enough to stop a 1000lb+ shelf from moving.
It can be muscled to move further, but obviously I would prefer it to be smoother.
The 2nd picture shows it's 'abundance' in the other rails.
I am trying to substitute the lack of steel reinforcement on the inside with something else. Not sure if something else exists.

 

[Komrade]

Here is another pic.

 

[Komrade]

This is a picture of a good connection

 

[juiced10]

I install movable aisle shelving systems for a living. The black pieces are just to keep dust/debris out of track. The wheels of the carriages should be riding on center rail. If the seam is off at joint we usually grind and feather out the joint.

 

[Komrade]

What kind of grind wheel do you use?

 

[juiced10]

Flap Wheel

 

[Komrade]

Thanks!
I'll give it a shot after Thanksgiving..
If I screw it up, i'll flip the rail and try to align those steel pieces.
I'll take any filler ideas I can also use to help with the 'smoothing' in the meantime