An "Easement" is an exception to the routing guide for tickets - where following the general rules laid down for which routes can be followed has been officially judged to be a bit silly.
For example,
30262
Customers travelling Southampton Airport Parkway and North thereof to Bitterne and beyond may double back between St Denys and Southampton Central. This easement applies both directions.
allows passengers from Southampton Airport to places like Hamble to change trains at Southampton Central, even though the it would be a shorter journey if they changed at St Denys, where the lines from Southampton Airport and Hamble come together for the final couple of miles into Central. The easement make sense, because lots of trains don't call at St Denys and making people change there could result in some very long waits, but the general rules don't allow passengers to go back and forth over the same line.
The list of easements is at
http://www.atoc.org/clientfiles/files/easements.pdf if you want to take a look; although it refers to updates (which occur from time to time), there's no date on this file. I would appreciate if someone with industry knowledge could let me know if it's the current version (and if not, where I can find the current version!)
Another sensible one:
30074
Journeys from Castle Cary, Bruton and Frome to Pewsey and beyond may travel via Westbury. This easement applies in both directions.
Allows passengers from the Somerset end of the Berks and Hants to change at Westbury for Pewsey. Through trains are allowed anyway, but as Westbury station is off the direct line, using a Weymouth line local into Westbury and changing there wouldn't be allowed without the easement.
Be aware that there are some negative easements ...
700042
Customers travelling from Castle Cary to Bristol Temple Meads and beyond may not travel via Taunton. This easement applies in both directions.
30234
Customers travelling from Edinburgh to Wellingborough may not travel via London St Pancras. This easement applies in both directions.
30219
Customers travelling from London to Heysham Port may not travel via Douglas Isle of Man. This easements applies in both directions.
700042 here is disappointing - to my untutored view, it would seem to be perfectly reasonable for people from Castle Cary to be allowed one stop to Taunton, then 1 stop on an express to get them into Bristol. Far quicker than a through but infrequent and local train with 9 intermediate stops.
Some easements have me scratching my head - seemingly allowing things that are pretty darned obvious. Are they there just to fix flaws in the way the general rules are set out, or have I missed something?
30114
Journeys from Melksham via Bedwyn may travel via Westbury. This easement applies in both directions.
30012
Journeys to or from South Greenford, Castle Bar Park, Drayton Green, Hanwell, Acton Mainline, Ealing Broadway, West Ealing, Hayes and Harlington and West Drayton to Reading and beyond may travel via Slough. This easement applies in both directions.
30020
Journeys from Moreton in Marsh, Kingham, Shipton, Ascott under Wychwood, Charlbury, Finstock, Combe and Hanborough to Reading and beyond may travel via Oxford. This easement applies in both directions.
30044
Journeys from Charlbury, Finstock, Combe and Hanborough to station north of Banbury may travel via Oxford. This easement applies in both directions.
30046
Journeys from Totnes, Paignton, Torquay, Torre, Newton Abbot, Teignmouth, Dawlish, Dawlish Warren and Starcross to stations north of Tiverton Parkway may travel via Exeter St David's. This easement applies in both directions.
300358
Customer travelling to stations on the Marlow and Henley on Thames lines via Didcot Parkway may travel via Reading. This easement applies in both directions.
300393
Customers travelling via Norwich using fares routed Irish Ferries may travel via Norwich.
I can't imagine - 30046 - how you could get from south of Exeter St. David's to north of Exeter St David's without passing through Exeter St Davids!
Suggestion - if you make a journey where you feel that the journey you're making is awkward due to railway geography or timetabling, search the easements and see if someone's been there before and added an exception to the rules in for you.
