There seems to be quite a bit of interest in having a higher level in recent threads so rather than answer one I'll give my views on the subject in general, my layout has twin levels and this idea came from my father who was I think inspired by the GNR at Halifax, anyway my current and recent lauouts have had a gradient and so here goes.
The gradient - Generally I have two double track ramps at 1 in 33, to arrive at this I did a lot of testing, it seemed OK as the real railway has grades of 1 in 40 - Somerset and Dorset, again 1 in 40 South Devon banks, 1 in 37 Lickey but then could go tighter to 1 in 19 Rochsoles, Marylebone down to Circle line, and others. I examined fathers layout and his was about 1 in 19 and it worked but it looked too steep, 1 in 27 Werneth incline Oldham, Exeter St Davids to Queen Street is 1 in 37 whilst Peco will tell you 1 in 36 is as tight as you should consider.
Going uphill is a matter of traction so a big train towed by a 4-4-0 will struggle at these gradients depending upon the loads of course but even so can climb light engine but the mighty Bachmann 9F will tow anything up a Werneth incline.
In fact when I tested this matter the biggest issue was coming down where droopy weak tension lock coupling jammed up, the old metal loop Wrenn couplings hardly noticed the gradient coming down,
On testing I tried a variety of other ideas such as using a Bemo/Roco type coupling and some other continental ideas and in the end discarded all except for the Bemo style and the dear old tension lock even the droopy type.
Overfilled MPD on the right shows the high level at 132mm above the baseboard level of the 4 track main line below