I could have brought home half a dozen bags of fast-setting concrete from site last night that would do a pretty good job. I don't know what it's proper name is but the track workers call it 'Boff' - it comes in small bags of about 15kg, mixes directly with water, pours, doesn't shrink and crack and you can run trains on it within an hour of pouring. Very fine aggregate in it but by God is it tough, I dread having to break it - even with the demo robot it's hard going.