4%

[bnsf971]

What I did back when I had HO scale, was use a double track helix (one track up, one track down). It only took up a few extra inches for the second track, versus two complete helixes. The trains went up the outside track of the helix (maximizing curve radius) and down the inside track. I had a small yard at the top level, with a coal mine. I used a crossing (I think it was 15 degree) to get the trains back on the correct sides after getting on flat level ground again.
About that 4% grade on a curve, my experience says don't do it. I lost an entire coal train onto a concrete floor because I tried it. The 25 broken hoppers didn't hurt as much as the three brass SD40s that were totaled.

 

[UP2CSX]

Terry, I can only imagine the look on your face when surveying the damage. Good advice from the voice of hard experience though.

BOC, there's no problem with running the train up and down the same grade, especially if the second level represents an industry and not another layout. The only issue you face is turning the train since you surely don't want to back down a steep grade. You could do this with a reverse loop or a locomotive wye and runaround track.

 

[JoeG]

after reading all these post I can tell that 4% is out but what about a 2.8% straight track with a few curves

 

[UP2CSX]

Joe. 2.8% is still steep but possible although you'll have to at least double head most of your trains. The biggest problem is getting engines to run at the same speed together both up and down the grade. If you're running DCC and Back EMF decoders, this is not simple but can be done. If it's straight DC, finding the right combination of engines can be a real chore.