English railways directed up past it (6)

I believe the answer is:

senile

I'm a little stuck... Click here to teach me more about this clue!

'past it' is the definition.
(I've seen this before)

I don't understand how the rest of the clue works.

(Other definitions for senile that I've seen before include "Old and doddery" , "Suffering from weaknesses of old age" , "Mentally weak due to old age" , "Relating to old age" , "past it" .)

I've seen this clue in The Guardian.

Want a hint initially instead of a full solution? Install my app