Building a helix for a model Railroad. this is not a how to video, but more a video that might give you some guidance if you are going to try and build a helix.
Biggest thing is Test Test and Test some more. making sure every level works perfectly before moving on to the next level. Yes it takes a long time, but this is going to be a part of your layout that you are not going to be redoing anytime soon. or just ANYTIME at all for that matter.

take your time cutting the parts, take your time building the levels, take your time laying the cork road bed. and take your time doing the track, test all solder joints. and did I mention take your time.
allow time for glue to fully dry, for all steps. and run a couple different locomotives, not just one, up and down.
only then, when it is perfect, can you move on to the next level.

it does get frustrating, but again take your time, walk away for awhile, come back fresh, it is not a race, it does not have to be done in a couple days.

if you are doing a helix, TAKE YOUR TIME and TEST