How Mk works: every vehicle has Mk2-6 tiers (better speed/cargo/power, same look + your colours). A mode's Mk nodes are an escalating ladder — 10 points to Mk6 (ranks 1,1,2,3,3), so most stop at Mk2-3. Your Mk ceiling applies to every vehicle of that mode, vanilla or specialist. Specialists (Coal Hauler, etc.) unlock once and then ride your mode's Mk. Each tier also arrives later in-game — Mk2 two years after the base vehicle… Mk6 a full decade — so you need the talent and the year.