Think you will find its to help stop the track moving, its bolted onto the sleepers and its driven into the ground to get a grip.
If you go to Hessle Road Junction at Hull ( Docks Branch) its has a load fitted to stop the line moving and been fitted for a couple of years now after the junction was changed from previous single lead to double lead. It was not long after been relayed that it started moving and then these were fitted to stop the movement.
Yep, They are "lateral resistance plates" or "lateral sleeper resistance anchors". They are made by vossloh. They are an improvment on the lateral resistance end plates and are for increasing lateral stability. This is especially improtant on sharp curves in CWR where high thermal forces are more likely to push the track out of alignment.
They are also used near S&C which can have high lateral loadings for diverging trains and are also used there track movement can be higher than normal, often due to formation heave effects.