I don't know, the RMT seemed pretty resolute in their response here -
https://www.rmt.org.uk/news/rmt-sets-out-six-key-reasons-for-leaving-the-eu/
This isn't about Brexit, but let's not kid ourselves that this is a union caring about employee welfare or it wouldn't be so emphatic in its points above, points which have proven to have been wrong and impacted employee pay and conditions. This strike boils down to pay ultimately, with a few safety trojan horses put in for good measure with this strike to balance public opinion. They'll get to the third strike and the settle on a 5-6% increase and move on.
The wider worry for the RMT should be the declining numbers of people using the railways, that is where job losses will come from longer term.