You can't go far wrong with Peco - Code 100 or Code 70.
Actually, I prefer the current Atlas flex track, although it may not be
available or economic in the UK. One rail is fixed to the sleepers, the
other slides, making it easier to maintain even sleeper spacing.
If you are doing curves the Hornby semi-flexible track is easier to work
with as opposed to their fully flexible track. Otherwise go for Peco if
you need concrete like sleepers and don't buy steel over time it will
rust in a loft.
It depends on what sort of layout (or really what sort of stock) you are
planning to run over it. For appearance I would recommend SMP's flexible
track, which is built with code 75 rail, and looks very good. The OO
track has often been mistaken for EM. If you are running older
locomotives, then Peco's Universal (with code 100 rail) is ideal, looks
not being too important.
If you want more realistic concrete looking sleepers try
http://www.intercitymodels.com/ICM1.html who sell representations of the
different manufacturers of sleepers and code 83 rail close 113lb flat
bottom rail only now being replaced by heavier UIC 60 rail.
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