Hobbies And Interests
Space Considerations
Modelers with large train rooms may have the space to create a mainline scene with long, high-speed trains. Those who live in small apartments may be lucky to squeeze in a 12-inch-wide shelf-type industrial scene with operation restricted to small switching moves. A traditional 4-by-8-foot tabletop is large enough to contain a grand scene in tiny N scale, but O scale will require curves so tight that long engines or cars stick out awkwardly and unrealistically.
Costs
HO scale (1:87 of actual size) tends to be the most affordable model train size since it is the most popular and mass-produced scale. N scale (1:160) is slightly more expensive. O and 027 scale (1:48), S scale (1:64) and the ultrasmall Z scale (1:220) are much more expensive, which may or may not affect their suitability for a given modeler. G scale (used for garden railways) and the other very large scales that share its track size are expensive as well. In the more-expensive scales, one may simply choose to design around fewer locomotives and cars.
Desired Operation
Each modeler has specific interests. Some people are endlessly fascinated by the "chess game" of shuffling two or three cars around complex yard tracks in a confined area. Others may want to run long passenger or freight trains simultaneously on multiple tracks, without switches or stop-and-start operation. The track plan should facilitate one's desired type of operation. Consider John Armstrong's "Track Planning for Realistic Operation" (published by Kalmbach Books) as required reading in this area.
Realism
Some modelers like to recreate with fanatical detail a specific time and place, say the Santa Fe Railroad in a small Kansas town in 1963. Others simply want a nice table with casual scenery and buildings for running a "toy train" with minimal realism. Either approach is perfectly valid, as is creating one's own realistic but fictional railroad (this is called "freelancing"). Realistic track plans should closely follow real-life trackwork, while a toy train layout can be anything desired.
Time Demands
It is important to know how much time one will be able to devote to a layout, both during construction and afterward. Maintenance is required to keep it in good running order. Tracks need to be cleaned and accumulated dust removed on a regular basis, or trains will run poorly. Keeping a too-large layout in good condition may exceed one's time and energy. A compact layout, perhaps in a small scale, may be the better answer in that case.
Further Information
Books and magazines in abundance discuss very phase of layout planning and building, from trackwork and electrical wiring to scenery and operations. Many model railroaders of all skill levels meet regularly to help and learn from each other (visit nmra.org). They also "meet" online in discussion groups and browse the hobby shop downtown and train show at the high school. Anyone wishing to model a real railroad can find history books loaded with prototype photos or discuss their plans, locally or online, with an historical society dedicated to that line.