the increased distance travelled by many trains highlighted problems with the fireboxes and chimneys. with the co-operation of the north midland railway at their derby works, he measured the temperature of the exhaust gases, and decided to lengthen the boilers on future engines. initially these "long-boiler" engines were 2-2-2 designs, but in 1844, stephenson moved the trailing wheel to the front in 4-2-0 formation, so that the cylinders could be mounted between the supporting wheels. it was one of these, the "great a" along with another from the north midland railway, which was compared with brunel's "ixion" in the gauge trials in 1846. in 1846 he added a pair of trailing wheels - the first with eight wheels. another important innovation in 1842 was the stephenson link motion.