Complex sentence: The first railroad track laid through Thistle was a narrow gauge line built in 1878 by the Utah and Pleasant Valley Railway, as a spur line to service coal mines near today's Scofield Reservoir.
Split into simpler sentences: The first railroad track laid through Thistle was a narrow gauge line, built in 1878 by the Utah and Pleasant Valley Railway. This line was a spur, to service coal mines near today's Scofield Reservoir.