There are suggestions doing the rounds that this structure wasn't built as a
water course, but as a 'subway' under Feltham Yard when first built. There
is a second course of the River Crane under the tracks further east, next to
the recently built PO sorting office.

As a non expert, I'm wondering if the foundations for a subway would meet
the requirements for a culvert.

Probably not. *There would be no requirement for the foundations of a
subway to be designed to resist the scouring action of flowing water.

Was there a plan to run the London LOOP walking route under Feltham
Yard? I remember having to make a long detour through Hounslow Heath
and through Whitton to get back to the river. It would seem a good
time to get a new subway built while they're replacing it, since I
doubt the LOOP could have justified it on its own.

Perhaps it was hiking extremists who caused the flood damage by
blocking up the other course!