You board a northbound Thameslink train to Cambridge, which is 10 minutes late going north from St Pancras (for whatever reason).
First of all, you have to get onto the GN at Belle Isle. This probably won't be an issue, although it could be if something happens to be leaving King's Cross on the down slow just at the right time - if so one of the trains is going to be delayed.
At Finsbury Park there's the choice of three lines. You will probably find a slot on the down fast although may have to wait a gap in the East Coast, Hull Trains, Grand Central or even Great Northern service, so may sit in the platform at Finsbury Park waiting for this. Alternatively you can run on the down slow but somewhere along the line you're likely to catch an all-stations Hertford or Welwyn train, you will also be constrained by the lower line speeds on the slow.
By Potters Bar the train more-or-less *has* to cross to the down slow, as you now have station stops at Potters Bar and Hatfield, as well as later the need to slow for the DF-DS crossover at Welwyn GC if you stayed on the DF. If you stay on the fast, you *will* be holding up something else.
But crossing onto the down slow now means the out-of-path train may well become tangled up with an all-stations Welwyn GC train, which calls additionally at Brookmans Park and Welham Green. The signaller may be able to hold that back, delaying that, depending on how the two services converge. Worst case scenario is you follow one all the way to Welwyn, possibly losing 5-10 minutes depending on how it pans out.
Whatever happens, by the time you get to Welwyn, the train now has to occupy the down fast again to get over the viaduct, as well as the call at Welwyn North. The need to call at Welwyn North means you will take up the equivalent of two paths, bearing in mind the booked two paths have been wasted. Unless you're very lucky and there happens to be a two-path gap just at the right moment, either you get held back for a suitable gap, or something else gets delayed. So perhaps another couple of delay minutes for someone.
From Woolmer Green you can run fast line or slow, but you're calling at Knebworth and Stevenage, so if you run on the fast line you're almost certainly holding something up (the same train you may well have been holding up behind you whilst you called at Welwyn North). If you run on the slow line there might be a terminating Hertford line train reversing in platform 4 at Stevenage, which could block you for up to 10 minutes, or alternatively that could be held back and delayed to make room for you. Also bear in mind that there are 4tph to Cambridge and Peterborough who also need to be routed one line or another through Stevenage and are subject to the same constraints. By Hitchin, everything needs to share the one down platform in order to make the call. Quite easy to accrue 5-10 minutes delay here for one or more trains.
Having left Hitchin, you're no longer tangled up with Peterborough services, but you're now booked to call at *seven* intermediate stations. Using the same line are 2tph non-stop London to Cambridge, and 2tph which call at three of the seven stations. There's a limited overtaking opportunity at Hitchin by using the old route (albeit a train using the old route will have to cross both up lines on the level), and an even more limited opportunity at Royston by routing something through the up platform (if one of the 6tph up services aren't using it at the time). Both these overtaking opportunities rely on the trains being in the right place at the right time. So either you get held back at Hitchin for however long required (if there's somewhere to hold you that doesn't delay something else), or something else gets stuck behind you and potentially delayed for 15-20 minutes.
Finally at Shepreth Branch Junction you have to fit in with 3 or 4 tph off the West Anglia route. If you're unlucky enough to coincide, one or the other will be delayed by a couple of minutes. One of the Anglia services actually runs all the way to Birmingham.
Bear in mind that 2tph are ultimately planned for the London to Kings Lynn service, these being the non-stop trains described above. This route has long single-line sections between Littleport and Downham Market, and between Watlington and Kings Lynn. Delay one of these services and your down delay has now potentially transmitted to the entire up service with the same domino effects right back to London.
The alternative scenario is your service gets re-scheduled to miss out stops, or turned short somewhere. Either of these options creates a 60-minute gap at many intermediate locations with the proposed 2tph service patterns.
Now imagine all of the above when there's more than just one late-running service emanating from Thameslink...
