On XBOX? Probably never, at least so far as a "huge sale" is concerned. A few random routes are occasionally discounted, but that's it. A selection of DLC for other XBOX titles is discounted every week, and we periodically get huge 'landmark' sales where an enormous amount of DLC is included, but TSW DLC is seldom included in these sales. I suspect this might be algorithm driven, e.g. maybe TSW players are less "price sensitive" when it comes to purchasing DLC and will buy it regardless of cost so there's no need to discount it. Against that is the fact that the likes of STEAM, and to a lesser extent Sony, don't restrict discounts of TSW DLC as much, but each marketplace will have its own policies. For me personally the lack of sales means that DTG (and MS) get less of my money, but I may not be a typical TSW player.
I've seen a preview of upcoming XBOX discounts in the Black Friday sales and the only TSW-related content included appears to be Train Sim World 6: Special Edition. There's otherwise no DLC that I can see.
