Originally Posted by
threeoh
I haven't tried myself, but a few FT threads indicate it's not possible:
I had no issues doing it about 2 months ago. I was in the UA terminal and they refused to let me catch the bus without a T5 boarding pass, but I went landside, caught the train, and was able to enter security without any issues.
Coming back there is no bus, so I had to again go landside to get back to the B/C gates.