My distant recollection is that drivers on national rail were 80%
ASLEF, 20% RMT but the ratio was reversed on the Underground with 80%
of drivers RMT, 20% ASLEF. The ASLEF drivers were concentrated on the
Northern Line and one other (which I cannot remember).

Is this approximately the position now? If so, why are TfL expecting
significant disruption on all lines? Surely they are operating a
reduced service anyway for a Bank Holiday?

Possible ironic tweet from Gary Lineker:

"WBA team bus is stuck in traffic trying to get to Loftus Road. They
should have got the tube…"
E.

Scott wrote:
My distant recollection is that drivers on national rail were 80%
ASLEF, 20% RMT but the ratio was reversed on the Underground with 80%
of drivers RMT, 20% ASLEF. The ASLEF drivers were concentrated on the
Northern Line and one other (which I cannot remember).

Is this approximately the position now? If so, why are TfL expecting
significant disruption on all lines? Surely they are operating a
reduced service anyway for a Bank Holiday?

Yes, I had the same recollection as you, so was surprised at the level of
disruption the ASLEF strike has caused today. Perhaps the problem is that
not many drivers were due to work anyway today, and if the numbers showing
up falls too far, it's hard to run a service.

I also wonder if TfL are all that worried about providing a good service on
a Bank holiday? The revenue would be small, and the costs of beating the
strike high (eg, how many managers qualified to drive would be prepared to
come in?).