Greased lightening
I have a 340 and the rubrail was a constant pain, because it was always dirty and nothing seemed to do a good job except products that were highly costic. I used some nail polish remover which took the dirt off, if also slightly melted a thin layer of the rubrail. But eventually, I got a product called Greased Lightening, which is a spray on, the dirt came right off with little or no scrubbing and it doesn't harm the rub rail. One problem with cleaning that rubber piece is that it's easy to also remove the wax on the boat. You can buy greased lightnening at an auto parts store. I bought it at AutoZone.ajp