Overall, I think TFL▸ is the wrong operator for these services and a national rail network operator would have been better suited. Like I mentioned, if TFL want to run trains on the national network, then they should follow the same rules as other operators, and the tried and tested methods used by the national network, rather than trying to apply their underground methods to a railway which is clearly interurban.
It's been a long-standing practice to extend tube lines by connecting inner suburban lines to them instead of their terminus. This was seen as relieving capacity constraints in the track leading to the terminus rather than in platforms, though of course it does that too. At the same time it prevented numbers on suburban lines dropping too far (which was a big issue in the 60s), by offering a direct link into the tube network.
The Bakerloo line north of Queens Park is the only place on the tube network that would be comparable to the
GW▸ mainline and this is a corridor with six lines. Both services on this line are now TFL run I believe. The district line shares with network rail from Richmond to Gunnersbury. again both services are now run by TFL. All the other tube lines appear to be run as their own closed network, which is of course much easier to regulate and operate than a shared railway. Some may have been former national rail lines but since the conversion to a tube line do not interact with any other trains. Comparing Crossrail to a tube on steroids isn't the best way of describing what it is and what it's supposed to achieve. It will be used like a tube in central london but a better comparison is a local train on steroids and even the Marlow train has a toilet.
An equivalent could be imagined I suppose if Wimbledon District line services continued from there along the slow lines to Basingstoke.
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