What do people use to fill joints in card models to hide the join, when say butting up two sections of platform. What is used to hide the join and make the platform look continuous?

If you're using a printed paving stone card or covering with printed paper paving, you could try cutting the ends to follow the joints between paving stones where 2  sheets join. If you want a tarmac or gravel surface then get a roll of belt sander, fine grade 320 grit or higher, never mind the colour, a spray can of grey primer will make it look like tarmac. These for example are 50mm wide and 1525mm long.

Thank you, but I should have said that I am using Metcalf models. My problem is where a ramp or staircase attaches to a building the top stair needs to blend in with the floor of the building, or the ramp of the coal distribution building needs to smoothly blend into the building, and I have shortened a platform by cutting a section out and sliding the two ends together giving a joint on the platform. I thought people might have a preferred filler or hard drying paste.

You're not really going to be able to do it like that because they're not painted.  I use 1mm mount board and squadron putty takes to it really well (only takes minutes to dry enough for sanding back), but then I prime and paint everything afterwards.  If you look around the forums there's losts of recommendations to go round the edges of Metcalfe kits with felt pens or crayons etc. but not sure if you can hide the joins really.  Not from what I've seen anyway.

Rich