t-shirt at the Thomas the Tank store, this one at the Ian Allan Bookshop, and this one at National Road Transport, and this enameled sign, and this one on eBay. We're not seeing any notice of copyright or a statement of license. That doesn't mean nobody claims ownership, but it feels like this content is in a category you could call free-range copyright. If owners exist, they don't enforce rights. It's also possible that the content is public domain, but determining that would take some digging into railroadiana.
Can I use the title? Even if the sign's content were protected, you can use the phrase "railway enthusiasts disease" without infringing. We don't see any evidence that it's being used as a trademark and short phrases are difficult to protect under copyright law.
P.S. As you've noticed, all of the examples are from British sites, so we're going to assume you're British as well. In that case, we issue our typical disclaimer: our answer is based on U.S. copyright law, which although derived from British law, is not the same.