My DD and I just did this tour on July 10th of this year. We were staying at the SOG...quite close to Poly and had the same question. We were told several times on the phone and at many guest service locations that it may be an issue...just like you have been told. However, we finally found out...that the RESORT monorail would be running at that time. We were told to be there at 7:15...we got there at 7:00...resort monorail was running AND there were TONS of people already in line to enter the park (not there for tours). The tour guide was also very gracious and waited until the last possible minute (7:30) to start so that everyone was able to attend even though we were told to be early. (There were some arrivals later than 7:15.)
This is a really neat tour. If you have ever considered doing it...I say GO FOR IT. I learned alot about Walt, not just the trains. You are actually allowed to take pictures once you get to the "round house" area (which is back behind the MK). We got some really neat ones. I hope that you enjoy it as much as we did. This was my 10 yr olds first tour and our guide was the best.