Am I the only one who can't figure out how to stack into the massive train engineer doll in the first area?  Anyone with a tip for how I can accomplish this? 
 
- Scott

I haven't got a clue, I just wanted to say that 'Unique Dolls' would be a great name for a band.

I figured it out! 
 
Just for anyone else who gets stuck, as you complete the chapters, more and more new dolls get added to the train station from the other levels. 
 
- Scott

I'm looking for a way to stack the Line Dude in the train station.  He won't turn his back to me.  Do I just have to wait? 
 
- just realized you can't stack into that guy at all.  There are wings off the side of the station!

I was also wondering how to get into both the huge train engineer and the "Line Dude" near the ticket booths. Now that I know more dolls come into the old levels I can finally move on.

You have to complete the second level first.  After completing the second level you gain the ability to stack more dolls. 
 
The big engineer is more than 1 size bigger than your biggest doll in the first level.  The game only lets you stack in to a doll 1 size bigger than the current doll you are stacked in. 
 
Basically the big engineer is a size 5 doll, and at the outset of the game you are a size 1 doll, that can stack in to a size 2 doll, that can stack in to a size 3 doll.  But you can't jump from a size 3 to a size 5 doll. 

As usual, they made a video of this on youtube, check out "achievement hunters".
 
( they also have Halo fails of the week, and Halo horse each week..... real fun to watch.)