In the end, even to an introvert, simply stating "Oh hello, I'm so and so, unfortunately I have a lot on my mind, I'm sure you understand" isn't outside the bounds of the reasonable. Do you disagree?

Personally, as quite an extreme introvert, I would probably not make any excuses to get out of the conversation, but I would wish they had never spoken up in the first place.

We live in a culture of extroversion, where transparent excuses to avoid talking to another person overwhelmingly tend to be viewed as rude.

While I sympathize with extroverts who would be discomforted by a long train or plane ride in close proximity with no conversation, starting one in a place where the other person does not have the option to physically disengage, even without applying intentional pressure against their doing so, does carry a risk of discomforting the other person.