One last question.. I understand having to have an accompanying diesel with large passenger trains to keep up the speed and in case of break down.
The possibility of a "break down" has nothing to do with why a diesel is involved. For excursions under the Amtrak insurance coverage, there MUST be a diesel with HEP capabilities (Amtrak usually specifies one of their own units) in order to provide 480 volt, three phase electrical power for the entire passenger consist.
Since most railroads no longer have passenger units , a freight unit is used?
Unless HEP is required for powering the passenger cars. See answer above.
... what is the maximum speed for most freight units say on the UP or Ohio central.
For a DC traction unit with freight gearing, 70 MPH is usually max, even though the overspeed function of the trailing diesel is not functional, as it is in the "trail" position. The RPM of the DC traction motor armatures should be the limiting factor, whether the diesel is under load or not.
Most CN freight units were limited to 70 per, Higher than 70 and a penalty brake was very likely very if no action was taken.
Correct, as the lead, controlling, unit would have its overspeed function active.
Anyway 70 is a pretty good speed. Can the 844 or 675 maintain a speed of 70 on level track with 20 passenger cars without the diesel?
I don't know what locomotive "675" is, but UP 844 would be more than capable of handling a 20 car passenger train at speeds well above 70 MPH, on LEVEL track. Referring to the Cheyenne Frontier Days Special passenger train, which is usually more than 20 cars, which is made up of many heavier business cars/domes, and the grades between Denver and Cheyenne, there would be no way 844 could have handled that train unassisted, and maintained the tight schedule.