Plaque

Andrew Motion was commissioned to write a poem for this garden.  It is cut (in a very interesting way) into the low concrete wall around a grass playing field in the garden:

Light licks its finger-tips and turns a page
of earth - this earth packed down beneath us now:
it gleams of Romans facing Boadicea,
flows over Chartists on their green-sprigged stage,
picks up a railway-tremor in a terrace row,
then leaps to hold a jump-jet in thin air. 

All dead, all living, all a concrete sign
of freedom learning how to find its aim:
to prove our lives our own - you've yours, I've mine -
and each one different but each the same.

The "railway" and the "jump-jet" both refer to King's Cross station.  Can anyone explain the "concrete sign of freedom"?