Hard bop is a mix between rhythm and blues and bop. It is more blues into the jazz of that era, with more focus on funky rhythms. By the mid 50s bop had played its part of being a very complicated musical genre, it was ready for a renewal. In the same year Miles Davis came with Birth of the cool, instigating “cool” jazz, Coltrane came with this early injection of blues into jazz.
Blue train was released in 1957, yet somehow its recording quality is way better than what you might expect from that time. This is one of the many Hackensack recordings from Rudy van Gelder, and not everyone is convinced about the musical quality. I am however.