London Euston of course used to have one between platforms 2 and 3, the ramp to it still there from a bridge over the lines about midway along the platform.
Looks like Preston may well also have had at one time - before the ticket hall was where it is now? Or maybe that's always been its location.
That cab road, with an exit onto Eversholt Street, was also used for
access to the station from Eversholt Street by vehicles carrying royalty when they joined the royal train from one of the platforms adjacent to the cab road.
It was on the old platforms 5 & 6, and disappeared when the station was revamped as the Eurostar terminus. Entrance was from the east side (Pancras Road) at the north end of the station and the taxis left through the arch in the hotel and passed down the front ramps into Euston Road. None of my books on St Pancras and other London termini say when the cab road fell out of use, but a diagram of St Pancras in 1958 from The Railway Magazine reproduced in Alan Jackson's book "London Termini" (David and Charles, 2nd Ed, 1985) refers to the "Former Cab Approach".
Yes, it seems obvious - and no doubt was the case for some time - that cabs leaving St P would use the front ramps down onto Euston Road. However, as I mentioned earlier, there was
definitely a ramp inside the curtilage of St P station, which taxis used to exit down to Midland Road. Perhaps that was mainly for empty cabs, which had arrived up the front ramp with passengers, dropping them off after going through the station entrance arch - which is now presumably the hotel lobby - to drop off by the booking office, before exiting down the narrow ramp with the sharp turn at the bottom.
So maybe taxis coming to
pick up used the old cab road, accessing from the north-east of the station, then exiting down the front ramp; and ones arriving to
drop off entered via the front ramp and left, empty, down the tight internal ramp. Though the flaw in that logic is how I would have ever been driven down that internal ramp if it was only used by empty taxis?
The main Eurostar taxi entrance and exit that I remember was under the building right next to the main pedestrian entrance to the station, which seems to be Mepham Street. Pending demolition it’s all boarded off but still visible on streetview. But that area between the station, York Rd, and Leake St seems to have at least two different road levels, and a loop at the west end, and there are also accesses to underground parking at the Leake St end. It’s all a bit of a maze, but as you say not really a cab road anyway.
According to Alan Jackson's London's Termini, there was an entrance to this space between platforms 11 and 12 for empty taxi-cabs from Griffin Street, off York Road, but it had been closed by the time he wrote the book. Was it still used by Post Office and newspaper vans? I recall seeing the top of the ramp in the distance when passing by the platform end on the concourse.
There used to be a couple of short bays at the end of platforms 11 and 12, capable of taking perhaps two carriages, which I assume were for vans. I never saw them in use. They were removed when the platforms and cab road were rebuilt with two new lines with new full-length platforms.
When the Eurostar station was in use, it seems that there was a taxi-rank at ground level under it, with an exit through Leake Street and round under the approach road to join it before it met Westminster Bridge Road. I think the entrance was off York Road at the other end of Leake Street. It wasn't therefore an actual cab road as it wasn't at platform level. It fell into disuse, and Leake Street is now purely a pedestrian passage way in which kids may spray their designs on the wall (well, it's much better there than on the side of trains).
Re Waterloo in Eurostar days. Yes, the taxi entrance for Eurostar was right next to the main steps on the Mepham Street side of the station. The "cab road" was indeed near to ground level, not platform level. But of course all the facilities of customs etc etc were underneath the international platforms, and so led easily out onto the taxi rank. When taking their passengers, the taxis had a compulsory left turn at Leake Street (not being able to turn right to get to York Road; can't remember why now). Leake Street led them under the tracks, to a
very tight and narrow right turn in the open air at the end, and back under cover to merge into the exit road from the high-level road on the south-east side of the station, then exiting onto Westminster Bridge Road (where it runs under the approach tracks) to turn left or right to the outside world. This exit - from the high level road, that is, not from Leake Street) is still used by some cabs going from their drop-off/pick-up places on the current "cab road" in front of some of the station offices, under the Waterloo East pedestrian bridge (ie more or less parallel to and above Waterloo Road). Of course cabs also - besides the Westminster Bridge Road exit - now have the option of running parallel to Mepham Street, going down to York Road from their ranks (though they can only turn left there), and the option of going down Spur Road (past the crane used to get W&C Line tube trains in and out) to Baylis Road.