From an old git-
On transition was used by AC CLN as a means of applying, but not actually applying in some cases, separation to streamed aircraft. They could then transfer the aircraft ASAP to TC East but not entirely confirming that they would both be flying at the same speed. I personally gave up ringing CLN to inform them that the following aircraft was actually flying faster and had they ever read the section about separation being 'constant or increasing'.
Parallel headings-Always fails safe.