From my memory of being involved in putting HTA's & 60's then 59's on the LBT/Fiddlers trains, 66's would lift 19 HTA's out of LBT, but were downloaded in the wet where as a 59 or 60 would happily lift 23, possibly more but constrained by the RR loop at LBT. The difference in loadings subsequently became apparent on running LBT to Ratcliffe services, when these were increased to 23 HTA's, a 59 couldn't get up Caverswell bank, but a 60 would.