Spikes don't hold rails down, they hold them in place.
Rails are not rigid, on purpose. they are susposed too give and flex.
Spikes hold them from moving side to side/rolling over---the weight if the rail holds it down---and---with a train on it--well, with a train on it.
I understand the fear--but there is little worry.
Now, if a rail breaks---OH WELL.
Blessings