"This easement applies in both directions."

Just a random thought I had.

Most routing guide easements, and all I have personally seen, are followed by the sentence "This easement applies in both directions".

Are there, or have there been, any easements that apply in only one direction (and if so why)?

Yes, these two at least:

700695 To facilitate journeys to Pilning station on Saturdays from Bristol Temple Meads, this local easement will allow the journey to be valid via Severn Tunnel Junction.

700696 For passengers travelling from Pilning on Saturdays to or via Newport South Wales, this local easement will allow interchange at Filton Abbey Wood station to get a connecting service back towards Newport South Wales

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Because since the very beginning of the Routeing Guide, permitted routes have been directional. Permitted routes from A to B are not necessarily the same as permitted routes from B to A. Therefore it's necessary for easements to state the directions in which they apply. Underlying this, both services and fares can be directional. Stations where on a particular day you can only travel in one direction, for example.