I had a trip that used two coaches, the first coach was without issue so I won't mention it, the second coach was awful, before we were allowed to board, the driver had a half hour conversation with another driver, then when we were boarding he refused to let me on because he claimed my two medium suitcases were large, they were not. I told him that by every definition including national expresses classification, they were medium suitcases, he threw a tantrum and brought it over to the chart which indeed showed that they were medium sized suitcases, he got increasingly aggressive as I informed him that the chart showed that they were medium sized. He continued his tantrum and said "well I'm the one who actually works here so I know what it is" and I replied "well clearly you don't" he was reading the hand luggage as medium undercarriage, and medium undercarriage as large undercarriage, I explained to him that if that were the case, why would it be labeled as carry on, his hissy fit continued and he went to get the manager, now I technically did violate the rules as I had a backpack and a carrier bag to take as carry on, as my carrier bag held my assistive technology for my disability, i hadn't considered it as carryon because I need that technology similar to how a blind person and their guide dog. When the manager confirmed my suitcases were medium sized and I could board the coach, the driver continuing his temper tantrum pointed out that I had two carryons, and refused to let me board, the manager was obviously had to side with company policy, but she was approachable and friendly about it, when she told me I could pay an extra £15, I told her no as it would be cheaper to take the train, the driver was unaware of this conversation I had with the coach driver, and upon realising I wasn't boarding he looked at me with a mocking smile. It wasn't about the luggage as most the people in line were boarding with over the designated amount of hand luggage, it was because he knew he was wrong and wanted to cling on to anything. Frankly I believed he targeted me over everyone else as I was a teenage girl travelling alone, and he was already angry at something and saw me as an easy target to release his frustration at. It wasn't really about the not being let on, it was about his abhorrent attitude and glee upon believing he left a teenager abandoned in a city with no way out. Frankly I believe that anyone with that attitude towards people shouldn't be in an occupation with that contact. I made a complaint to national express, and although eventually they did respond that the driver could have been nicer, they reaffirmed what the driver originally said about national expresses policies that one large suitcase or two medium suitcases is the luggage allowance, which i was well aware of and wasn't the reason i wasn't allowed to board the coach as i was in the right in the arguement over suitcase size, as confirmed by the manager. As my ticket was standard I was denied a refund for the coach I was never aid to board. I have began to use Megabus as an alternative, although not the greatest reliability wise, I am yet to have a driver as incredibly rude as this one from national express.