On Jan 26, 3:21 pm, (Neil Williams)
wrote:
No. The fasts leave Euston on the fast lines and don't cross to the
slows until Ledburn (I think) while the slows leave on the slow lines

How fast is fast? Do fast LM trains that don't stop at LBZ (e.g.
london-milton keynes-wolverton or london-bletchley-milton keynes)
cross over there?

The crossing for LBZ fasts is certainly between LBZ and Cheddington.

[1] The 1824 does not stop at Leighton Buzzard or Berkhamsted, which I
always suspected to be a crowd control measure given that the xx24 and
xx54 of other hours does.

The 1823 departure is fast to LBZ (then MKC, Wolverton and
Northampton). The 1824 stops at Harrow, Watford and a few other
places. I was on it once, having arrived at HRW to see a "northampton"
train arriving at the platform, jumped on, then realised as we sped
through LBZ that I was on the wrong train.

Got to Bletchley and waited for a train back to LBZ, I guess
technically I should have been Penalty fared. Twice.

Between the 1824 and 1834 the big stations are covered.