You are confusing the quality of the existing track with the quality/speed of Amtrak's service over that track.

According to Amtrak's November 7, 2011 timetable, Amtrak #4 departs LA at 6:15, stop in Fullerton at 6:50 PM, stop again at Riverside at 7:33 PM, stop again at 7:59 at San Bernardino, then stop again at Victorville at 9:10 PM. They're taking 2 hours and 45 minutes, with four stops, to cover those 120 miles. Each time the train stops, it loses a lot of time because the train has to decelerate, it then spends dwell time at the station as people get on and off, then the train has to accelerate back up to speed.

So, yes, if you look at Amtrak's current timetable between those two points, that train is really slow.... but that's not the fault of the track. Consider at Metrolink's speed over the same route. Per Metrolink's schedule for their San Bernardino line, you can get on their train #310 at 4:06 AM and arrive at LA Union Station at 5:30 AM.... that is 1 hour and 24 minutes to cover 60 miles. That works out to an average speed of 42 mph... which doesn't sound fast, until you note all of the stops that the train makes between the LA and San Bernardino... specifically:

• Rialto
• Fontana
• Rancho Cucamonga
• Upland
• Montclair
• Claremont
• Pomona (North)
• Covina
• Baldwin Park
• El Monte
• Cal State LA

In order to make all those stops and still average 42 mph between the end points, Metrolink trains have to really zip between stations.... they do that using the existing 79 MPH track (I rounded up to 80 MPH in my earlier post) between LA Union Station and San Bernardino.

I hope this makes sense.