Complex sentence: The track itself, until taken over by the LMSR, was mostly secondhand; because of this, the line was dubbed the "Slow Mouldy and Jolty Railway" by travellers.
Split into simpler sentences: The track itself, until taken over by the LMSR, was mostly secondhand. It was because of that that the line had been dubbed the "Slow Mouldy and Jolty Railway" by travellers.