I’ve never understood why GTR simply runs this service as a Dartford-Rainham shuttle, could they not somehow divert this service via Bexleyheath or even run it as far as Charlton to serve Blackheath & Lewisham then to Elephant & Castle via Peckham Rye?
Often it’s likely because the Bexleyheath line (or the North Kent between Blackheath (where it converges) and Lewisham are closed. Or because the slow SEML lines between St Johns and LB are closed. GTR can and do divert via the Bexleyheath line, albeit GTR drivers don’t sign the Sidcup line.
Dartford generally offers an easy interchange and good onward connections via SE services.
Whenever the Greenwich line has been closed this service gets diverted to Blackheath and Lewisham and then takes the slow/Cannon Street lines to London Bridge, aren’t drivers not cleared to run Thameslink trains from Lewisham to E&C then I take it?
No, they don’t sign the route from Lewisham via Nunhead and Denmark Hill. What would be the point of them signing such a circuitous route when they can go via Lewisham - London Bridge? That’s following their usual route a lot more closely and quickly. On the occasions GTR can’t use either direct route to LB, people wanting London Bridge/Blackfriars will be much better off travelling on SE services to LB via another route, changing at Dartford for those coming from further south.
EDIT to add: if the route via the SEML slows is blocked, for example if Charing Cross or CST are closed, many SE services are often diverted to Victoria. This puts a lot of extra pressure on the Catford Loop line around Denmark Hill, another excellent reason not to send GTR Rainham services that way.