I forget now if it was here that the suggestion be put that it be converted to a raised monorail system, to serve both sides of the border.
The same E/W-ish tracks would exist as a spine to connect both the Statesward and Estadward 'feeds' and stations, but 'capsule control' on the train units would normally allow only transit between the same country for source and destination, either deep in each territory or at "wall-top" stations (you'd have either passing-loop or pocket-track "platform options" with doors and barriers that only open to the assigned access/egress direction).
If you board at an accredited "International Station" you step into a cross-border unit, whether that's just a short trip along the wall (e.g. Mexico maintains an IS at the east of a split community, the US at the west of it), a short trip over (Tijuana North spur onto and off again to San Diego South), a longer one (Escondido to Ensenada), or even a long dog-leg diagonal (between Veracruz and Vegas, as an express that travels almost the full border, between and two long spurs).
The 'wall' can be mere support pilings where there's no need (or easy ability to plant) continuous groundworks, including down the Rio Grand, as one long viaduct (there's
precedent, give or take an inversion), or where you'd otherwise create defacto exclaves, and you could equitably
split the track around sacred Indian lands or other awkward sites, rather than bulldoze straight through them.
As well as passenger trains (though perhaps blending in with them, maybe even coupled to them) you'd have border-patrol units using the frequent transits to train both Mk1 eyeballs and cameras over areas that need it (and now places that are solid enough barriers have a further tricky top to cross, anyway).
Of course, it would be solar-electric (tack the panels onto the structure, however best done, putting power almost straight into the system, though obviously needs grid/storage capabilities, might even allow further cross-border interlinks - except Texas if it still wants to go it alone) and might even at times be a net source of exportable power (from border, both directions) as had already been suggested with some of the initial long-list Trump-era suggestions.
As a joint project, it would give a green-industrial push on both sides of the border, and lead to new opportunities (and, yes, new challenges) in something of a Manhattan-/Apollo-project vein, but different to both. It could be extended and integrated with other travel systems to create The Great American Link throughout that multicontinental landmass from Tiera Del Fuego to Battin Island, if not Resolute and onwards. (Possibly much further, but it seems the Bering Straight is hard enough, as the 'easiest' of the barriers needing to bridge to connect with the inevitable pan-Afreurasian network that will cover the larger part of the planet and its population.)
I'm sure you will agree that this is an emminantly achievable task. 'Tis but a Simple Matter Of Engineering, of course. Now, we'll just take a brief coffee-break, and I'll be taking questions from the press first, then I'll ask if you can leave so I can open it up properly to the attending members of the public who are here to express their interest in investing...