As a tip, you can buy cork floor tiles from woodies. They pretty much match up exactly to rail height. Am thinking of experimenting with using them as hardstanding, then filler between the tile edge and the track... You can see a similar method here: http://www.rmweb.co.uk/community/index.php/blog/1/entry-44-inset-trackwork-on-keyhaven/
On the upide, its a lot less filler, and should be easier to get an 'expanse' of flat, level surface (you can always spread filler thinly on the tile anyway before painting too)