Styrene. Go to your local

Styrene. Go to your local store or the interweb and buy sheet styrene. Easy to work with and a lot more forgiving than plaster 

Styrene alternate: EVA foam

Others have used sheets of EVA (craft) foam. You can get it in various thicknesses. They run around 30 cents a sheet 8"x11" at Wal-Mart here.

plaster of paris or styrene ?

Either will work, I've also used masonite to represent concrete. The main thing is finishing it in a realistic color and with weathering suitable to your era and location. Thinking back to RIP tracks I've seen it would probably need some dirt and grime from brake shoe dust, axle bearing grease,  etc. unless your are modeling a new modern facility? From what I recall the SP RIP track at Watsonville jct. was mostly gravel or cinder covered but might have had some concrete jacking pads at the rails? . .....DaveB

I have used EVA foam, buy

I have used EVA foam, buy black and paint with a sponge to an asphalt colour. The nature of the foam makes it look a lot like asphalt. If you buy 1mm thick, it can be tucked under the rail head and allows operation without cutting a flange space, so it looks like asphalt right up to the rail.

Place mats

Look at the back side of Dollar Store place mats.  Many of them have a textured finish that can be painted to look like packed earth or some sort of pavement.

Dollar Store place mats.

  Thanks for the tip.. The  $ store has lots of modeling supplies, I get wire strainers and storage containers for scenic material  and their foam core is handy if I need a piece of thin foam sheeting, the paper facing will pull right off leaving bare foam that can be shaped for roads, parking lots etc. ...DaveB

R.I.P.

Oh dear... I didn't even know Mr. Apron was sick.

Pavement

I've used both styrene and Woodland Scenics "Smooth It" for pavement areas. Large areas, for me, are easier with styrene, but I really prefer the look of the Smooth It when finished, it just has a more realistic concrete look in my opinion.

Mike in Florida

asphalt/concrete

These pads would far more likely be poured concrete, not asphalt. Asphalt would be a little "soft" for jacking up cars for repair and would buckle and crack after a while.

Thanks all

appreciate all the replies.  I will look into EVA foan as well as "Smooth it"

Ken