I DVR'd this film and watched it last night.
Someone else on here pointed this out, and I agree, that Uncle Charlie was not the widow murderer. Were there tons of circumstantial evidence? Yes, loads of it, but we never see Uncle Charlie do anything until the end, and then, I think he's finally at rope's end. Reluctantly, he must dispose of "Girl Charlie". He could never have a moment's peace if he allows her to live. He knows that only too well.
Remember, Uncle Charlie is kind of insane anyway (although, no murderer). There was the scene at the bank where he shows his a$$, and then the other times when he goes on one of his tirades.
Hitchcock goes to great lengths to NOT show Uncle Charlie doing anything wrong. We never see him lock the garage door or turn the car on. We never see him put the key back in the ignition. We never see the entire inscription on the ring. We never see him booby-trap the back stairs. (Yes, we see a figure from the back that may be Uncle Charlie, but we're not sure.) The only bad thing we see Uncle Charlie do is throw a glass against the wall. Big deal. Why was the other suspect back east "running from the police". Maybe he was the murderer.
I believe Uncle Charlie got a raw deal getting crushed by the oncoming locomotive. Let him get missed by the train and then sneak away unnoticed. Then, we never know whether he kills again or if he was innocent. The ambiguous ending is always better. I wish I could have consulted Hitch on some of these things.
Remember at the beginning on the film how cynical Girl Charlie was? Before her uncle even showed up? Maybe Girl Charlie is on her way to becoming just like Uncle Charlie? Especially now that she's become so embittered from that ordeal.
One thing I'd like to add is, I don't like the romance angle with the detective. Unnecessary, underdeveloped, and completely unrealistic. Not a good move by Hitch and the writers. This fault keeps 'Shadow' from getting into my Top 5 Hitchcock pictures. Which is:
5) The Man Who Knew Too Much (1956)
4) Rear Window
1) North By Northwest