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Originally Posted by electricron
All excellent points, although I believed you missed one, i.e. station spacing. A train can achieve a higher average speed if it doesn't stop as often.
Comparing average speeds between MARTA to DART.
MARTA Red Line North, Five Points to North Springs, 11 stations, 29 minutes, 17.65 miles. Averaging 36.5 mph.
DART Red Line North, Union Station to Parker Road, 17 stations, 45 minutes, 21.15 miles. Averaging 28.2 mph.
DART Red Line North, Pearl to Parker Road, 13 stations, 36 minutes, 19.64 miles. Averaging 32.7 mph.
The difference between the DART examples is that one is eliminating one and a half miles of the street mall in downtown Dallas. DART's street mall is 1.5 miles in length, 4 stations, and 9 minutes, the trains averaging a fairly low 10 mph through it. There's a huge difference in the average speed of DART's light rail trains if we include the street mall. Where most of the slowdown occurs is when running at grade level in city streets. But light rail trains within a dedicated corridor can average close to the same speeds as metro trains, the average speeds differential depends mostly on station spacing.
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I agree. That however is where express trains shine. If on the Red Line, for example, trains going in the same direction could pass one another, then, a "Minibullet Express" might average the 19.64 miles at 45 or more mph, for an average of 26.2 minutes. An express train with an average speed of 60 mph (which would be very fast for a commuter train) would take 19.64 minutes. Lower priority express trains, can be set up to hit every other, or every third station, giving the express option to all stations on a line, assuming that 1/2 of the stations have the same direction train passing option (there is a bit of scheduling math here, but the 1/2 rule is a good yardstick)
Express trains work best as the length of the line increases by lengthening the transit comfort zone, which, IMO, is around 30 minutes travel time. The more distance that can be covered in 30 minutes, the greater the potential draw.
Express trains also provide the feeling of speed.
Express trains lengthen the distance between stops on rail lines with distances a shorter distance apart.
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A truly great commuter system would have high speeds on dedicated right-of-way and a hierarchy of express trains supplementing local trains. An express train with 120 mph peak speeds on well designed right-of-way and stations with the same direction train passing option, conceivably could average 90 mph. At this point, the train would cover 45 miles in 30 minutes which is far faster than can be done by auto. An East West express train link between Dallas and Fort Worth, would cover the 36 mile distance (by car) in 24 minutes, and, cover the 80 mile east/west Metroplex in about 53 minutes.
Light rail then could hook into the express line at transfer points.
Someday some metro area will do this, most likely overseas.