amogles

I would be inclined to say it depends on the relative length of the
line.

If for example, the length of quadruple track is relatively short, and
it ends in a terminus, trains coming in on the slow lines will most
likely go out on the slow lines, and ditto for fast lines, so grouping
by speed helps reduce on conflicting movement and so boosts the
overall throughput of the layout.

Sorting by direction makes more sense for longer lines, however, as
the switching of trains between fast and slow lines can be acheived
without having to cross other lines. Again, this has repercussions on
line capacity and service punctuality.

Of course these problems can also be solved by flyovers but these
require considerable capital investment, which is probably only
warranted when none of the alternatives are acceptable.