This is an interesting question! I've done my best to answer by comparing the gauge requirements to the ones for containers:
Firstly I dug out some lorry dimensions for a haulage company with a range of mostly curtain-sided trucks that will do as a representative real-world example from the UK:
https://huntstransport.co.uk/our-fleet/dimensions-and-capabilities/
And here are some dimensions for the larger (taller) varieties of transit vans:
https://www.vanguide.co.uk/guides/ford-transit-dimensions/
Taking the transits first, the heights vary from around 2.5 to 2.8 metres. This is comparable to the difference between the standard container height of 8 ft 6" (2.6m) and high cubes at 9ft 6" (2.9m).
The width quoted of 2.474m is very similar to a container, which are usually either 8ft (2.44m) width for deep sea containers or 2.5m wide for European ones.
From this you can assume that it wouldn't be too difficult to transport the taller models of transit vans around on the same routes you can take containers, without the wagons needing a lower deck than a typical container wagon.
Transit vans built at Swaythling (and later imported) were a familiar traffic on rail from Eastleigh and Southampton, although the wagons used would perhaps not have allowed the taller models:
https://www.flickr.com/photos/shawfordjunction/40083602872/
https://www.flickr.com/photos/solent-rails/10176904605/
https://www.flickr.com/photos/71092681@N07/16634761329/
Some container wagons (I'm thinking of the IFA / FIA "multifret" design with a deck height of 945mm) are already in use by STVA for car traffic, fitted with flat-decks which are mounted like containers. In fact here they are with transits loaded onto them:
https://www.flickr.com/photos/50619197@N07/24769628785/
If the deck fitted raises the height to something slightly below a metre above rail, the taller models of transits (not loaded in the picture) would probably end up creating similar gauging restrictions to W10 container traffic, although being vehicles rather than containers they would need to run under "X load" conditions (which is already the case for the regular flows of cars in double-deck wagons). A slightly lower deck design (similar to the multifrets) would allow traffic on a wider variety of routes, similar to W8 container gauge.
As can be seen, the problem is that even the ubiquitous types of van that are used to deliver Amazon parcels are already close to being the same size as containers that require gauge enhancement beyond the standard W6A.
Now for the lorries...
The haulier mentioned earlier has 7.5 ton GLW curtainsiders which are 3.5m tall, 12 ton curtainsiders which are 3.75m tall, and anything bigger than that is listed as 4.1m or 4.2m overall height.
Comparing this to intermodal container traffic, a typical "standard" intermodal flat has a deck height slightly below 1 metre above the rail, which can carry containers up to ~2.9 metres tall within W10 gauge. So we can imagine that, at most, we have 3.9m to play with for our vehicle traffic to operate on the same network as the core W10 intermodal network (which is already somewhat restricted in terms of routes).
Ignoring special wagons with small wheels (FLA, IDA with deck heights between 700 and 730mm), "pocket" wagons (KTA) and the FAA well-type wagons, the lowest flat-deck intermodal wagons in widespread use are the "megafret" types (IKA, FKA) with an 825mm deck above rail height. If you imagine some similar wagons were built with a flat deck suitable for vehicles, deducting the 825mm deck from the total available height of 3.9m leaves a little over 3m for the vehicles themselves, which is hopeless for lorry traffic.
To get around this, combinations of specialised wagon with specialised trailer have been used in the past. Apart from the "Piggyback" design mentioned above, there was also the "Eurospine" design for which special Parcelforce lorry trailers (capable of being lifted on and off by crane) were built. They did operate in regular service between Mossend and Willesden for a while.
Quite a sight on the WCML (not to mention the 37 + 92 combination at the front)...
https://www.flickr.com/photos/75784477@N08/52211365229/
And looking suitably surreal at Warrington:
https://www.flickr.com/photos/wagonsontheweb/9205064139/
Extra height is gained by having the wheels of the trailer positioned below the top of the rail wheels. However, you are restricted to lifting on the trailers (or in the case of Piggyback, other awkward manoeuvres which are a far cry from the "rolling motorway" concept of Le Shuttle). By the time you have built a special wagon and a special lorry trailer (well below the typical height of ones used for parcels and mail) and lifted it onto the train at an intermodal terminal... you might as well just be using a container, or driving a regular lorry the whole distance.
Another road-rail system was trialled by Blue Circle but was very short-lived. Although cement tankers had far less problem with gauging due to their round profile, I think there was still some sacrifice of payload compared to a conventional cement lorry, added to the hassle of loading and unloading. Looking at a picture of the train, I assume these also must have been lifted in and out from the wagons. The caption is rather cynical about the motives for the trial, but it does make for a lovely picture, like something from a model railway:
https://www.flickr.com/photos/97660049@N06/31605741071/
The wagons ended up in Europe before reappearing in the UK for a brief period, used as low-deck container wagons on an Immingham - Doncaster Iport trial service:
https://www.flickr.com/photos/74356787@N05/51739730798/
https://www.iportrail.com/humber-express-shuttle-service-launched/
I'm not sure how well reported it was in the railway press, but there was a one-off trial of lorry trailers from Cologne to Barking (Ripple Lane) container terminal via HS1 for TX Logistick in June last year:
https://www.railjournal.com/freight/london-cologne-intermodal-service-to-launch-in-september/
The wagons used have some resemblance to the general design of the Eurospine discussed above:
https://gueterwagenkatalog.dbcargo....gory/T3000e-Sdggmrss-T3000es-Sdggmrs--9121260
An unusual feature is that the system is designed to load standard lorry trailers by placing them in a sort of "cassette" for loading onto the wagons. Pictures of this can be seen in the railjournal article above, which says that
"The service will be able to carry semitrailers using the NiKrasa system, which allows non-craneable semitrailers to be loaded onto pocket wagons without the need to adapt wagons, trailers or terminal facilities."
In the end I don't think there was enough demand for the trial to bring it into regular operation, although it says the system is in use in Europe operating between Germany and Italy.
With a typical lorry being 2.5m wide (the same width as a European container) it can be seen that along with the height, width would be an issue for any sort of "drive-on" service for lorries. To fit within the loading gauge it seems inevitable that lorry trailers would have to be securely mounted in a similar way to containers, rather than "parked" on a wagon with any room to spare each side. I also wonder whether those T3000 / Sdggmrs wagons would fit the regular UK gauge even when empty - they are wider than the lorry trailers they carry which might foul the UK loading gauge for station platforms, for example. Ignoring that possibility, the info appears to say that the trailers are carried a mere 270mm (0.27m) above rail height.
Going back to the roughly 3.9m above rail to fit within the W10 gauge for container traffic, that would still only allow trailers of about 3.6m height, lower than the 4.1m or 4.2m listed on that haulier website. It's hard to imagine getting the trailers much lower to the rail, so UK gauging will always be an issue no matter what wagon type is designed for lorry traffic.
To work out similar back-of-the-envelope calculations for other gauges (as requested in the original post), this is a very "rough and ready" guide to gauges as they relate to container traffic of a similar 2.5m width to the average lorry:
Max Height from rail for 2.5m wide containers to fit different gauges*:
W7 gauge = 3.393m (W7 gauge created for 8’ high ISO containers on standard flats)
W7a = 3635mm for 2.5m wide boxes
W8 gauge = 3.618m (W8 gauge created for 8’6” high ISO containers on standard flats)
W9 gauge = 3.715m (W9 gauge created for SB1C 9’0” high 2.6m wide European swapbodies)
W9A gauge = 3.730m (W9a gauge created for S45 swapbodies (2.9m) on 825mm deck megafrets)
W10 gauge = 3.891m (W10 gauge created for 9’6” high 2.5m wide boxes on most standard height flats)
W11 gauge = 3.891m (W11 gauge created for 2.55m wide 9’ 6” swapbodies on certain wagons)
W12 gauge = 3.896m (W12 gauge created for 2.6m wide 9’ 6” swapbodies on certain wagons)
*N.B this is a simplified digest compiled from various sources and the figures are not 100% accurate. There are some slight variations with different wagon types. The “J tables” in the rule book determine which combinations of wagon and container fit within each gauge. Gauging is not calculated purely on height, it also takes into account suspension characteristics and other engineering calculations which I don't pretend to understand.
It can be seen that moving "up" through the gauges does not always mean a big increase in height, as some are focused on allowing for wider European "swapbodies" with non-standard dimensions. Even a universally applied W12 gauge would be of very limited benefit for any sort of "rolling road" for lorries, which would require dedicated infrastructure...
Before there was HS2... there was the "Central Railway" freight proposal!
