L shape Lionel tubular layout engine slows down certain areas.... HELP..

No.  I never run my trains fast enough to derail on curves, especially not the highly detailed models.  Most locos today have speed control.  So even if a loco uncouples from the train and keeps going, it shouldn't speed up to the point of derailing. 

And that brings me to another sore point... the gear ratios.  Most of our models are geared too tall!  You have the biggest layout I know of.  You MIGHT have a need for more than 60 mph on your long straightaways.  For the rest of us working with a couple of 4x8's, two feet per second is plenty fast.  Using a lower gear ratio would prevent damage in those instances when a command control loco DOES take off at full speed.  From reading these boards I know it happens sometimes, and the current "safety" programming doesn't seem to prevent it. 

Finally there's the option to put halt / panic buttons at strategic locations around the layout.  Cut the power and they'll stop no matter what their on-board circuitry is telling them to do.

I've been to the New Jersey Hi-Railers layout twice and saw the frustration with my own eyes.  Even our 20 x 28 club layout was sometimes plagued with stalling because of weak / lost signal.  I stand by my original recommendation: put an OPTIONAL setting in the software (or a jumper on the circuit board) to keep going at the SAME speed if you lose the signal.  Some of us would be better off, and nobody would be worse off.  My $.02.

If both locos are having the same problem in the same area perhaps your  track is out of tolerance - outer rails bent slightly inward.

Spray some WD-40 on a rag.  Wipe (rub a lot) the track in the bad section with that rag.  You might feel the rag go from dragging to slippery.  Wipe all the WD-40 off very well with a clean, dry rag:  Nothing else.  Run the conventional engine again.  Any difference? 

If your running TMCC, one track over or under another, it’s signal issues my guess. I had this issue when one of my TMCC engines ran under another TMCC engine many years ago..We solved the issue with ground plain wiring. Running a wire from the middle screw of an electrical receptical to a terminal strip. From the terminal strip we ran wires under the upper level tracks and that flattened the signal causing the problem.  Give that a try... 

If your running Conventional engines, my theory may be incorrect.  You might have issues through the turnouts? Also, maybe the uncoupler tracks? Wonder if your track is new/ used...New transformers, older transformers? Lots of things can cause poor running of our trains.  Sorry if I gave you bad information...Good luck.

I ended up  today eliminating one  uncoupling track and I put extra power and grounds in between all the switches  and it works better  I do think its a problem with the ac motor  (old school) and tight turns at this point BUT it is not bad since running the trains will be more  hands on  than  just watching and not operating..

Does your train stop near or under the metal bridge? If so, this is where to place the ground plain wire...

no  never seems to of lost signal  ... but what was strange was it ran on the upper level at any setting .. but on the lower flat section  in would not hold a steady slower speed... but now  as long as I set it to run somewhat  smooth it s ok     thanks

Sorry about your issues, however, that’s a nice layout and very interesting to look at. If you could show exactly where the slow down happens, we could be of more help. 

Did you ever put a level on your track?  I'm willing to bet the track is also climbing at that point.  AC-motored locos are pretty sensitive to loads and grades.

I will check  cant  be that far off but I could be wrong I'll check   .... the bench work was level  see my previous posts on  new attic layout

Sorry if I missed it, but just want to make sure you are running a 14ga ground bus wire around your layout also, with drops every so often. If your switches have fiber pins in some of the outside rails, you possibly are not sending the return current back to the transformer through both outside rails from all areas between switches. As you know, return path is as important has the hot path.