Claire Jessiman, who writes the Foodie Quine blog and is based in Aberdeen, was travelling to Edinburgh by train when she asked the ScotRail staff member with the catering trolley to fill her reusable coffee cup.
The blogger stated that she was shocked when she was then told that she had the "wrong kind of reusable cup" and the staff member would not be able to fill it.
Instead, the writer was offered coffee in the usual disposable container.
Taking to Twitter after the incident, Ms Jessiman posted: "Excellent work @ this surely beats leaves on the line. Apparently I have the wrong kind of reusable cup so you can’t put coffee in it.
"Useful suggestion from the trolley staff was that I can now pour it in myself #
Ms Jessiman explained she would be the first to admit that she doesn't always carry a reusable cup, though this was the first time that she has been refused service with one.
She said: "Upon refusing to fill my cup, the staff member suggested that after purchase I could then pour my coffee from their disposable cup into my own reusable cup.
"Completely defeating the whole purpose and surely much more unsafe."
Responding to the Tweet, the official ScotRail account replied: "Hi Claire, The only reusable cups allowed are the 12oz 'Keep Cup' brand, this is for safety reasons."
When prompted by another user on what these safety reasons were, ScotRail added that the aforementioned brand "fits securely" into their pouring dispensers.
Other train operators such as Virgin Trains and CrossCountry have confirmed that they have no such restrictions in place on their services.
Unsatisfied with their response Ms Jessiman pointed out that the staff member in question did not use the dispenser to fill the disposable cup offered instead.
She added: "The stated safety reason by Scotrail to me on Twitter is that the Keep Cup is the only one that fits securely into the trolley dispenser.
"However their staff member didn't even place the disposable cup in the dispenser when filling it. I didn't have a ruler to hand but from the naked eye the base of my reusable cup is pretty much the same size as their disposable one. If safety at the time of serving really is the issue, surely cup size is the determining factor not a specific brand."
Responding to the original, Twitter users were quick to condemn ScotRail for the rule, and there was added confusion as to where the ruling was displayed.
Fellow food blogger Kellie Anderson added: "Wouldn't it be fab if they thought long-term on this, redesigning their serving kit to accommodate different sizes? Can't be beyond them, surely. Pretty sure they'd get great kudos for it."
A ScotRail spokesperson stated that the rules regarding reusable coffee cups would be added to their website, they said: “We recognise that we have a responsibility to reduce our carbon footprint, and accepting reusable cups is one way we do this.
“Reusable cups come in all shapes and sizes. KeepCups have been tested and can be used with our on-train catering equipment without risk of injury.”
When the total ban on reusable cups was lifted by ScotRail last year, the only stipulation seemed to be that travellers had to have a mug holding precisely 12oz.
Ms Jessiman added: "Apparently only one specific brand of reusable cup is permitted to be used on a Scotrail train, this brand so happens to be available to purchase complete with Scotrail branding from the trolley.
"In the current climate of attempts to reduce single use plastics this policy creates unnecessary barriers and I sincerely hope it will be reviewed and common sense can prevail."
According to recent report, it is estimated that the UK throws away around 2.5 billion disposable coffee cups every year, with less than 1 per cent of those being recycled.
It's not the first time that ScotRail have courted controversy over coffee. In 2012, they came under fire after it was discovered they had cut the size of their hot drinks by one-third but cut the price by only ten pence.