Question

Why was railroad construction concentrated in the east before 1870 and the west after 1870?

Answer 1

The main reason was the high population of the Eastern regions.   Government had lesser money to spend for the construction of the railroad system.   The western regions were less populated and the government had surplus money to build the railroad system.   Growth of cities in the 19th century was another factor behind construction of railroads in the west after 1870.