RMT union members at Unipart Rail are in dispute with bosses over pay.Engineers, clerical and production staff at a railway supply company will start a four-day strike on Tuesday in a dispute over pay.The Rail, Maritime and Transport union (RMT) said its members at Unipart Rail, who are based in Crewe, were angry that the company has been unwilling to improve a 4.75% pay offer which the union has rejected.