Susan probably made it back to Narnia, because her experience with faith reflects that of Lewis himself.
In 1960, C. S. Lewis wrote back to a reader and said that Susan is not in Aslan's country.
Not because I have no hope of Susan ever getting to Aslans's country, but because I have a feeling that the story of her journey would be longer and more like a grown-up novel than I wanted to write. But I may be mistaken. Why not try it yourself?
Source, provided by BESW in chat
"Oh Susan!" said Jill. "She's interested in nothing nowadays except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown-up."
This implies that she's starting on her 'silly phase', to paraphrase Artemis, which she will eventually grow out of and redeem herself.
Now, if you claim that Susan died in the train accident, The Last Battle doesn't say that. It makes no mention of Susan near the crash:
"There's not much to tell," said Peter. "Edmund and I were standing on the platform...
...
"...probably on the same train though Lucy didn't know about it -"
This is actually quite like Lewis's own experiences.
In Inside The Lion, the Witch, and the Wardrobe by James Staurt Bell, Carrie Pyykkonen, and Linda Washington, it's mentioned several times that he struggled with religion and then became religious (partly due to an all-night talk with j-r-r-tolkien and another friend).
Don't forget this quote:
When I was ten, I read fairy tales in secret and would have been ashamed if I had been found doing so. Now that I am fifty I read them openly. When I became a man I put away childish things, including the fear of childishness and the desire to be very grown up.
-Wikiquote
So, he looks like he's saying that he did the same thing that Susan did: Became 'grown-up', but then when he actually 'grew up', he was more 'childish'.
As @BESW stated in chat, this is sort of an intentional fallacy - this only is the case because CSL decided that this is the case, without there being proof from the work.
I've tried to find examples from the books about being 'grown-up' being a bad thing. I've only found a few things so far:
He didn't call his Father and Mother 'Father' and 'Mother', but Harold and Alberta. They were very up-to-date and advanced people.
The Voyage of the Dawn Treader, chapter 1
...
"Oh Susan!" said Jill. "She's interested in nothing nowadays except nylons and lipstick and invitations. She always was a jolly sight too keen on being grown-up."
-The Last Battle, chapter 13
So, it's entirely possible that she makes it, he just didn't feel like writing a 'grownup story'.