RAIL - seems to be leaning more towards business/finance now over the past few years...
RAIL EXPRESS - good all-rounders
TODAY'S RAILWAYS UK - another good all-rounder,especially for DMU/EMU history, line guides, improvement news etc (also a EUROPE version produced)
RAILWAYS ILLUSTRATED - probably the best for heritage/preservation and haulage, but good for theat only (poor on other levels). And the editor has serious problems with a certain Mr Hill, which I can't do with
MODERN RAILWAYS - what the title says, very engineering focussed though.
Overall:
General - TRUK/EU and RAIL EXPRESS
General and a bit o' business - RAIL
Heritage etc - RAILWAY ILLUSTRATED
Engineering - MODERN RAILWAYS
A good (free!) online one is Railway Herald, published every Monday, quite good for passenger news etc and rail tours.