I was sort of excited by the proposed difficulty of showing a transition into Narnia without showing the train wreck (BRAINSTORM TIME!). I was also struck by the difficulty of letting the viewers appreciate that what was a few minutes for Tirian (between his dream and Jill and Eustace popping up) was a few weeks for the Friends of Narnia-- without cutting away from Tirian and his plight for so long that you forget that was even going on.
Here is the (very unpolished) idea I came up with:
The movie starts with the Friends of Narnia gathering for a dinner together and then the apparition of Tirian appears. They ascertain that something is wrong in Narnia, and a frantic montage ensues of them putting together plans, Peter and Edmund breaking into the back of the Ketterleys' old house and finding the rings, and the others boarding the train to meet them. And... then we cut to the beginning of the story from the Narnians' point of view. I don't really have any ideas for an artful scene transition there; I'm sure you guys can think of some.
Later, when the Narnian story reaches the point of Tirian having his dream, we see the Dinner Visitation scene again, only this time from Tirian's point of view. Somehow this has to be radically different so that you don't feel that you're watching the same scene for a second time. Maybe, for Tirian, the dream is shorter and much more confused, with some blurred colors and garbled, indistinct voices. Some very different music and perhaps Tirian's loud, confused breathing will also help differentiate this scene from the one you saw at the beginning of the movie.
Tirian is thrown back, waking up once more tied to a tree and surrounded by the silent Narnian night. A long moment, in which he and the viewers have a chance to think, "What in the world was THAT?" before, suddenly, there are Jill and Eustace tripping over each other out of thin air and landing with cries of surprise.
So... that's my idea. This setup would allow you to see Eustace and Jill's entrance from Tirian's point of view, which I think would be the more fun and which doesn't reveal the train wreck. I think it semi-decently portrays the strange discontinuity of time between Narnia and England. It also allows the viewer to understand the English side of the story without it being told verbally, as it is in the book (but which is rather boring in film). And, I think the gathering of the Friends of Narnia would be a better opening scene than something in Narnia. I think I would pulled into the story more by characters I'm familiar with and a mysterious apparition than by characters I'm unfamiliar with. Plus, it sets the mood nicely: There's something wrong in Narnia and we have to get there.
Of course, you would still want to show the train wreck somehow... I suppose Jill or Eustace could make a small comment to Tirian about how their transition was very jolting, "I almost thought it was a train wreck," and then actually show snatches of the train wreck much later, nearer the end of the movie. I actually wrote a short fanfiction years ago that showed Jill having a vision of the train crash and her death in England as she was being thrown into the stable. I never posted it on here because, 1) it had some war violence and, of course, some not very nice pictures of death, and 2) it wasn't as well-written as my fourteen year-old self thought it was.

Goodness, that was long. I think it's pretty close to my original reply. *sigh of relief*