Ordinarily, I'd suggest that you start a separate topic and tell you how, but since the original poster used such a broad topic and I'm not going to be a lot of help anyway, let's just go with what we have here.
If its the same pin-striped, soft cloth train engineer's cap that I found a picture of, it's pretty neat. Its the sort of thing that would appeal to both Disneyland and train collectors. I honestly don't know the market value of your Viewliner conductor's cap, but in the right auction or collectibles sale, I'd expect it would be worth somewhere between $50-100.00. Perhaps a bit more. The Viewliner was a short-lived attraction that ran for about a year between Tomorrowland and Fantasyland starting in the summer of 1957. The fact that it's a Cast Member costume piece that comes from a vintage attraction adds to the cachet. The only thing that would detract from it's collectibility would be the fact that there will only be a limited number of collectors who would actually have seen or ridden on the Viewliner, so you have little chance that someone would be recapturing a childhood memory. And, of course, condition is important, too. If it's frayed or dirty, that will hurt quite a bit. I hope this helps.
