The shop tables I use are 1/2" tops, 4' X 8' with 6" sched. 40 legs and 3" sched 40 H bracing between the legs.  All are on heavy casters for various configuration.  One is 10 years old the other is 5 years old.  If I had the time and money I'd go with 5/8" minimum but they're still completely servicable although they do need to be looked after from time to time as in checking for flatness.  If you're going to be doing much railing you really need a flat table or you will be building warpage and bow into the rail.  When arranged end to end these tables allow for a 20' length to get built with little difficulty and just a couple of feet overhang.  However, every time I start a large project when I'll be doing a lot of repetitive railings I string the tables to check for flatness.  3 strings tight across a piece of 3/8" plate at both ends and a piece of 3/8" plate to check string clearance until I'm satisfied.  There are some adjustable screw legs underneath [some 1" sq. tube with 3/4 rod/nut adjustment] that get turned up or down to take out whatever bow has creeped in.  It works, but as said, heavier plate would do better.  I like the use of channel or W beam for framing as others have suggested, but these tables were built to a design from a job many years ago that worked well for that but didn't need as much strength as a heavy fab table should have.  I would make this reccomendation: be very conservative welding the table top to the frame.  The thinner your top, the more conservative you need to be because even small welds in a concentrated area of a large plate will draw a lot more than you'd expect and pull the plate out of flatness.  Best if you use very small welds, less than 1/2", in a well thought out welding sequence and keep them accessable so you can cut the tacks if you find the top draws too much.  There is no real need for heavy welding of the top to your frame, the load is static and the top won't be going anywhere after light welding.