tripleb,
First, it's my understanding that there is NO MK bus from the Outpost Bus Depot (up by the check-in building) so that is not an option. If I am wrong someone will correct me. You can catch a bus there to the other three parks, Disney Springs, water parks, etc. but not MK. All transportation for it leaves down by the lake from the Fort.
Second, internal buses start running IIRC a little before 630am in the morning but they are, how would I put it, ..... infrequent ...... in the first 60-90 minutes of the day. Then they become more frequent and during the peak times like 9am-9pm they are not long at all.
Third, you can bus to MK but it involves catching an internal bus (or walking) to the Settlement Bus Depot down by the lake, transferring to a bus that carries you to the Wilderness Lodge, then transferring AGAIN to the WL bus headed to the MK.
I don't know what type of site you have reserved but if it's a Premium then you want a loop down by the lake (700 or 400 or 600) and I would recommend walking to the boat dock in the morning and catching the first 630am small boat that will take you to the Contemporary and then walk to the MK entrance (which is where we met years ago when I did the Steam Train tour). Preferreds are already close to the lake. If you are in a Full, Tent, or cabin, I would be out at the local internal bus stop at 615am at the latest.
I would highly advise walking to the dock though because that is something you can control. At most from loops 1100-1200 it's a 15-20 minute walk at a normally slow walking pace. And be on the dock at before 630am. So leave your spot by 6am.
As we outlined recently in a thread for another poster, driving to the MK parking lot is a pain now with the changes around the MK toll booth; thus driving is not an option.
Bama Ed
PS - we did the Steam Train tour when we did it years ago but felt they dragged it out at the end trying to fill time with trivia. But you will love the opportunity being behind the scenes provides to get up close and personal with these mechanical beauties. Hope you will like it as much as we did.