For a straight platform I have used a long piece of wood, I wanted something that would be dead straight, robust, that I could screw and unscrew - drill holes into for lighting and cut out subways. I used 10mm thick wood and and for the platform supports I used Peco platform edging - this attaches well with super glue but you have to cut and sand the backs flush (they come with peg joints).
This Brings the platform edges 2mm higher up than the wood and I filled this in with plaster which I would paint to represent concrete.
This is where the problems began because the wood was not well keyed for the plaster - I would recommend un planed timber which is rougher and use a ready mixed filler such as you get at Wilkos - which is easy to work with - or instead use sandpaper (less messy but then you will have joints)
For the white line I used an Edding white pen and cut a v notch into the tip - and I was able to produce a consistent edge very easily.
Some of the photos I have uploaded show the results - the others were lost in the server crash