You could try oiling them once they are clean. This would be a more expensive option though, teak oil or danish oil is quite expensive if you're doing a large area. The only other thing would be to paint them every year with a decent quality wood paint, then you can choose a colour that suits. I'm not sure if they're still doing it, but cuprinol were doing free paint samples on their website earlier in spring (free p&p too, so I bought a few colours at £1 each to do my bird table - yep, cheeky I know!) Basically you need to weatherproof them so the fungus spores can't take hold. Sleepers are hardwood, so should last a long time, but even hardwood needs to be treated yearly when it's outside. I know weathershield works on brickwork, but not sure if it will work on wood.
If you don't want to bend to scrape the stuff off, then wolf-garten do a great tool for cleaning weeds off patios, that might be good (the knife variety, not the brass brush). I've used mine for scraping Ivy off my walls where I can't reach, I think they're fantastic tools, so you only buy the interchangable heads with a single handle. No, I'm not a rep for wolf, I just think the tools are very well thought out and made, and are great for me where storage is an issue.