The February 1986 one was one of the most comically stupid ideas ever conceived by British Rail, and was the reason why the 1987 and 1988 ones were scaled back.
Basically it went like this - any ticket up to £15 = £5
Any ticket up to £30 = £10
Any ticket over £30 = £15
I am sure you can see the inherent problem with this ...
So, *any* ticket was £15. So if you had a friendly ticket office person, or went to a BR travel agent (who frankly, usually, didn't have a clue what they were doing), it was "Hello, kind sir/madam, can I have a return from Penzance to Wick, via Brighton, Norwich, Fishguard, Holyhead, Hull, Stranraer and Aberdeen please?"
Instant All-Line for £15!
Even if you didn't have a friendly person, you could buy straight returns anywhere, so the usual one was to buy a return between two stations that were a long way apart but frankly no ticket collectors outside that area had ever heard of (Causeland to Plockton, please! And a return from Alexandra Parade to Alexandra Palace while you're at it). And then there were the funny ones (Bat & Ball to Bowling, Whitchurch to Whitchurch via Whitchurch).
Not only that, but the return halves were valid for a month, so if were judicious buying your tickets, most of March was covered as well!
Since it was the winter, there obviously wasn't the "rare engine" bashing opportunities available, but many people availed themselves of the excellent opportunities, I myself spending most of the two months blasting up and down the ECML and sampling the steam heat 47 action in Scotland (I cleared four boilered locos for 1000 miles, one of which was an English one that the ScR had kidnapped).
The Southbound overnights from Scotland on a Saturday were, shall we say, "busy", one memorable occasion on 1E43 with 47596 from Aberdeen to Newcastle via Carlisle, and 47401 from Newcastle to KX (via every conceivable diversion route - Leamside, Lincoln, Cambridge) having pretty much every metre of floor space filled with sleeping people.
Unsurprisingly, there was a rethink.