Newer diesel generators typically run at 1800RPM/60Hz, while many older ones ran at 1200RPM/60Hz (or less) A modern 50Hz diesel generator runs at 1500 RPM, an older one at 1000RPM, etc.

While your small portable generator may run at 3600 RPM, the generators in a power generator station such as this do not. Although 60Hz for 60 seconds is 3600 cycles, this does not determine generator rotational speed. Rotational speed is determined by the number of poles in the generator. A 2 pole generator will make 60Hz at 3600 RPM, a 4 pole at 1800 RPM, a 6 pole at 1200 RPM, an 8 pole at 900 RPM, etc. At one of the generating stations I work at, our steam turbine turns at 4458 RPM, goes through a gearbox (with a reduction of 4.953:1) and turns the generator at 900 RPM.
Everything EE mentioned is spot on.