Model railway tracks fall into two categories, raised and ground level.
Although many people run on ground level for the smaller gauges, even down to 2 1/2" gauge, my personal view is that raised track is best for 3 1/2" gauge and below, and ground level for 7 1/4" gauge and larger.
5" gauge can be either, and whilst there are some excellent 5" ground level tracks which adhere closely to prototype practice and standards,  I prefer raised track for this gauge, unless the model is of a narrow gauge prototype.

Raised track is more stable as the centre of gravity is lower.
Ground level tracks can have points (difficult with raised track) which enables a more complex layout to be constructed, and they look much more realistic.

Rail can be aluminium or steel, and in the latter case, is sometimes made of square or rectangular section.
Steel rail wears well, allows track to be welded, and, size for size, will take a heavier loading, but unless it is in frequent use, will quickly rust, and when next used, the rust particles will be lifted off the track as a fine duct which will adhere to the locomotive and rolling stock.

Aluminium rail has to be supported at more frequent intervals, especially under joints, and needs less
Maintenance. 
It is often claimed that a given locomotive will haul a greater load on steel track compared to an aluminium one, as the latter tends to be "slippery".

Ground level track.
Ground level track that has pointwork needs to be constructed more carefully than  raised track.
Multi-gauge track requires even more care, and must be constructed as a piece of precision engineering.
Knocking a few nails in to hold the rails in place is likely to lead to disaster.

For small curves, i.e., those close to the minima in the table on page 21, the gauge may need widening slightly, but no more than 3/32" for 5" gauge track.

There have been some excellent articles on tracks and construction, in Model Engineer, and for anything more than a basic straight or oval, it is worth referring to them

To make 1 cubic yard, of  Concrete requires:-
0.86 cu. Yd. Aggregate,
0.43 cu. yd. Sand
500 lbs. cement.
Water should be the minimum necessary, approx. 6.75 galls per Cwt of cement is usually sufficient.

Concrete = roughly 100 lbs. / cu. ft.
Compressive strength can be up to 600 lbs. / sq. in.
Concrete reaches "working" strength after 28 days.