These old railway sleepers have now been banned for sale to the public in recent years by garden centers due to the creosote they contain.
I have one in my garden which seeps tar too, even in the winter, if the sun is strong, the sun just seems to draw it out, I dont think you can do much to stop it, I usually throw a hand full of saw dust over it if I think of it.
Some garden centres now offer an alternative, usually oak, same size etc without the banned chemicals, maybe replacing it / them could be another option. Good Luck T bird+