Ormskirk and Kirkby both have four trains an hour, plus an irregular service to Preston/Manchester on what is generally considered a separate platform in the same way as at the likes of Bristol Temple Meads (3/4, etc) or Birmingham New Street (10a/10b, etc). An interesting point though is that the Merseyrail Lime Street station is actually operated separately to the rest of the station (Merseyrail manage it, not Network Rail), and is essentially only considered a part of the main station for logistical purposes. So in that sense it could actually count, with its 14tph off-peak. Of course, Moorfields and Central do not count, because there is the small matter of the Northern Line above it.
I'm surprised no-one has said Cardiff Bay yet. Along with the afore-mentioned Stourbridge Town, it is surely one of the top two, counting terminating services as two (arrival and departure)?
In terms of different services, Colchester Town must be in with a shout, having Colchester to Walton-on-the-Naze, the local to the main station only, and also a Liverpool Street train?
(Seen as I've mentioned them, one fact about Kirkby, and two about Moorfields: 1, Courtesy of the change of PTE stations passenger usage recordings for the 2009/10 data, Kirkby is now the busiest single-platform station by usage, beating Chafford Hundred by almost 300,000; 2, To access Moorfields station from Moorfields itself passengers have to go up an escalator from ground level, and then down from the ticket office to the underground platforms; 3, The gap between the two platform levels is unusually large for a simple "cross" station such as Moorfields, because whilst the Northern Line passes over Queensway, the Wirral Line passes under it.)
