If you want to operate your railroad in a realistic manner with a team of friends, and not just build scenery and models, you will want a large layout. A multilevel layout doubles or triples the length of the main line in the same room. Decades ago, a central panel overseeing the entire railroad was necessary to run a large model railroad; but remote controls have developed over recent decades which enable an operator to walk alongside his train. This makes multilevel layouts, with their curving, intimate aisles and hidden cul-de-sacs, feasible.
Decide how the tracks will connect one elevation to another. One way is a hidden helix. The other is a long slope, completely visible.
Design a helix with as large a radius as will fit, and as gradual a slope as possible; this will help keep trains, which the helix keeps hidden and inaccessible, from derailing. A double helix has a track for going up and another for going down. Helixes take up a lot of room in HO scale, much less in N scale. An O scale helix would probably be found only on a club layout. Use the center of the helix for a dispatcher's desk, so that the space does not go to waste.
Connect the two levels with a long, visible slope, either along a long wall, or on a peninsula. This is more accessible for troubleshooting than a helix, and it can be scenic instead of hidden. Dave Adams' On3 Durlin Branch, set in 1939 Colorado, is one example.
Increase normal aisle size from 2 feet to 3 feet, to accommodate more operators than a single-level layout. Calculate maximum grades carefully to avoid building a railroad only to discover when it is finished that trains cannot get up the hill: 4 percent for short, steam era trains. 2 percent for long, modern trains. 3-3.5 percent for a helper district, where extra engines must be on standby to assist trains ascending a hill. A helper district is a big headache for real railroads, but it is a highly desired feature on a large model railroad, because it increases the challenge of operation.
Read the track planning books of John Armstrong, still in print. He designed numerous dual-level layouts, some of which feature platforms and backdrops placed so that only one level can be seen at a time (his "mushroom" plan). That type of design requires a high ceiling, preferably 9-10 feet, higher than most Midwestern basements. Back issues of "Model Railroader," "Railroad Model Craftsman" and the annuals "Model Railroad Planning" and "Great Model Railroads" contain numerous descriptions of multilevel layouts, as well as additional articles by John Armstrong.
Tuck lights for a lower deck behind a valence, to keep them from glaring in operators' eyes. Use incandescents if you want colors and nighttime effects with dimmer switches; however, they are hot and large. Florescent tubes are cooler and easier to hide. Or, use strings of Christmas tree lights, which take up little room and can be easily stapled into the underside of the upper deck. Jack Burgess' Yosemite Valley Railroad has a photo showing how lights and wiring are crammed into the underside of an upper deck.
Build the lower level with conventional construction methods. Screw the cabinetry for the upper levels into wall studs or new studs in the middle of the room. Do not block the scenery between decks with support posts. Except in N scale, some hidden metal support, such as 90 degree brackets, is desirable for added strength.