That really is rubbish. Cambridge and Stansted must be the two primary destinations for folk changing from the ECML at Peterborough.
Interchange between the ECML and East Anglia at Peterborough has been significant ever since the long distance direct trains via the Joint Line stopped running in 1973. In 2025 I have spent too much time looking at the relevant data from the flows matrix.
Cambridge is easily the most important. It has one way flows over 10k for all of Edinburgh, York and Leeds. I estimate that the total for Cambridge to/from all ECML destinations is more than 50k. This means that, of the passengers who board a Stansted-Birmingham train at Cambridge, more change at Peterborough for the ECML than go through to destinations Stamford-Birmingham.
The Norwich flows are just a bit more than half the Cambridge flows, with Edinburgh, York and Leeds all about 6k.
The mix to/from Stansted is different, more dominated by shorter journeys. For Stansted, Lincoln is third biggest ECML destination after Leeds and York, and for Lincoln the Stansted flows are bigger than Cambridge or Norwich.
And a route that can get overlooked is Ipswich and Bury St Edmunds. I reckon that, of the passengers arriving at Peterborough on a train from Ipswich, up to half are changing for the ECML.
However, the numbers are still small when compared with the ECML flows to/from Kings Cross.
It is also worth noting that Peterborough itself has about 45k journeys to Leeds and about 40k journeys to each of York, Newcastle and Edinburgh. What we don't know is how many of those have driven quite long distances across East Anglia to get to Peterborough. My hunch is that it is in the thousands.
Connections at Peterborough between LNER and Cross Country services for Cambridge will surely be impossible and anyone travelling from Cambridge to Leeds, Newcastle and Edinburgh will probably be better off connecting with the Leeds and Newcastle trains that call at Stevenage rather than waiting an hour at Peterborough.
This could be a blessing in disguise. There is a lot more capacity between Cambridge and Stevenage than between Cambridge and Peterborough.