Huge writes
We already have Thameslink services from Brighton to Bedford. When the
Bedford electrification is extended northwards, as surely it must be
in the end,

Although we will likely all be retired by then.

And Hell will be quite a chilly place... I can't see electrification
being extended northwards.

Even if it was, it would be trying to mix two different types of market
on the same service - which doesn't work. People travelling form
further north will be wanting high speed trains which make few stops.
The existing services need fast trains which make regular stops - the
current service won't have many people travelling all the way from
Bedford to Brighton, but will allow people to go from Luton to Gatwick,
St Albans to Croydon.

So merely extending the existing trains won't work. Running separate
through 'InterCity' trains wont work either as the central section will
never have the capacity to accommodate both types of trains.

--
Dave