Bit strange why someone has to confirm the curtains and back doors work. Isn’t that what they should do anyway fella. Or are they famously hard as hell to pull and hard as hell to shut? Just asking?
You never worked in a fleet with over 300 curtainsiders? There are ALWAYS a few that are a pig to open/close, especially the older ones 
Worked at a firm with more than that but the garage lads use to spray the curtain rail with spray grease each service. Rarely had a tough one, I’d also keep a can in the cab as a lot of others did to keep on top of them. Winter is the only time they got a bit tough, must be the salt.