This appears to be an issue about the distinction between tramways and light railways. Tramways are tracks built into a flat surface, almost always a roadway, level with that surface. In Simutrans, tramways are tracks that can be built over road tiles.
Light railways may or may not include elements of tramway, but are generally more inexpensively laid, can take a lighter axle load load, often have sharper corners, often have less expensive (and therefore weaker) bridges and, by reason of the foregoing, usually have lower speed and weight limits than main line railways. Light railways are sometimes standard gauge and sometimes narrow gauge; are sometimes urban and sometimes rural.
Tramways, meanwhile, can be used with light rail or heavy rail: although most tram networks are urban light rail networks, there are a number of instances in the UK historically where main line (heavy) rail have used tramways, such as the Weymouth Harbour Tramway:
Class 33/1 33107 - Weymouth Harbour Tramway. by
Martyn Hilbert, on Flickr
In Pak128.Britain-Ex, we have a tramway way constraint intended for light rail tramways, and then a special type of heavy rail tramway that lacks this constraint to allow construction of heavy rail tramways such as the Weymouth Harbour tramway.
The functional definition in Simutrans between the tramway and the railway menu is between tramways (whether for heavy or light rail) and railways (whether heavy or light). In other words, the tramway construction tool is intended specifically for that type of way that is built on top of flat surfaces (usually roads) so as to be flush with those surfaces, whereas the railway tool is intended for that type of way that is built in open countryside. This reflected in the graphics; for tramways, only the lines of rail are shown, whereas for railway (whether light or heavy), the sleepers, ballast, etc. are shown.
The "green tramways" shown are, without exception, built, not in open countryside, but immediately adjacent to roads, on road beds, and the grass is turf added after the construction of the tracks. They are also a peculiarly modern invention. Their construction, and no doubt their economics, are fundamentally different to the construction of light railways in open countryside.
The purpose of the limitation on building tramways not on roads is to avoid the absurdity, common before the restriction was encoded, of having long tracks in the countryside of ostensibly embedded rail where there is nothing but bare earth into which to embed it. In reality, it would not be possible to build such lines.
The confusion here, I think, is likely to have sprung from the historical naming of the tram menu option in Pak128.Britain, a leftover from the Standard version from the 2000s when it was first created, which names this menu, "Tram/light rail tools". This is probably an erroneous name, which has remained simply because nobody has so far thought to change it, for, whilst most tramways are light railways, some tramways are not, and many light railways are not and do not include any tramways.
I can understand those wanting to build light railways incorporating some elements of tramway may wish to use one menu rather than two; but the people wishing to build a railway network incorporating both heavy and light non-tramway rail would then have to use two menus instead of one, so the overall ease of use for all players would not be greater.
The only reason to have a separate tramway waytype in Simutrans-Extended, rather than simply using way constraints, is to allow the building of rail lines on top of roads. Blurring the boundaries between tramways and light railways when, as explained, a light railway may or may not include tramways and a tramway may or may not be a light railway, is not ultimately a means of making things easier for users, especially when the boundary between light rail and heavy rail is often blurred in reality (e.g. a somewhat lightly laid branchline off a main line that occasionally has through traffic from the main line but has to use somewhat lighter engines than would otherwise be the case, or an urban railway line, such as the Metropolitan Railway, now considered part of the London Underground, an urban light rail network, but once was just a short main line railway company, the transition between which was subtle and incremental, and the distinction between which even now being more in the electrification type, rolling stock interior layout and service patterns than in the nature of the basic infrastructure itself).
The fact that there is a clear and precise distinction between tramways and railways but a much less clear and precise distinction between light rail and heavy rail is a sufficient reason to have the conceptual boundary between tram and non-tram rather than between tram/light rail and heavy rail, and on top of this there is the game function distinction of tracks that can be laid over roads, as well as the different economics of laying track on virgin terrain compared with laying track in a pre-existing road bed, which difference it is important to simulate. I would suggest that it is necessary to remove the reference to light rail in the tram menu in Pak128.Britain-Ex to make this distinction clearer.
I hope that this clarifies matters.
