That video looks very much like a water cannon in action not sandite.
The water cannon is used more than Sandite these days.
It's probably doing both. MPVs like that are 'modular', the modules (the boxes/tanks on the MPV) have different functions, e.g., a water tank, jetting module, sandite module, de-icing module (for third rail). Certainly all the MPVs I've seen in operation today (and there have been quite a few today- NR have been busy!) have been jetting the railhead then laying sandite - the sandite is visible on the railhead, particularly once it's dried, and there's a definite rumbling sound until a few trains have passed over the treated rails. Not sure what the forecast is, but if it's predicted to be a cold night then most likely they will have been laying de-icer on the 3rd rail too.
Funnily enough, driving the first train following an RHTT before the rail/sandite has had a chance to dry off a bit can be quite difficult - the railhead can be quite slippery making it difficult to take power without wheelslip, requiring a gradual increase in power and maybe a bit of hand-notching.
As for weedkiller, I think it's mostly glyphosate (i.e., Roundup, as available in the garden centre) used these days which is fairly benign (for humans at least) - most of the residual herbicides that were commonly used (e.g., diuron, simazine) have been banned/withdrawn or are frowned upon for their persistency in the environment.
edit: a quick google suggests NR may be using Pistol, made by Bayer, which is a 'non-hazardous' formulation of glyphosate + diflufenican (a residual persistent for up to 6 months). But glyphosate is only effective on actively growing plants so is rarely used at this time of year when plants are beginning to senesce.
