Hard to describe but it is a flat spring, sort of snail shaped and it holds the rail in place but has a bit of give to it. Someone should be along shortly to give it it's proper name.
Not sure of its proper name, but basically it’s a “key” made of spring steel, like so:
...that fits between the rail and the “chair”, as the baseplates are generically known, fairly old technology these days where rail is concerned. We move a lot of pre-assembled panels of line, anything up to 100’ long, (makes for an interesting journey considering that most rail access points are quite tight to say the least.... Always a joy to have 2 escort vans AND the Police in attendance ), and most of them use sprung wire “Pandrol” clips that hold the rail to the sleepers.