CE 515 Railroad Engineering
Source: REES Module 6 & An
Enhanced Parametric Railway
Capacity Evaluation Tool
“Transportation exists to conquer space and time -”
Factors Affecting Capacity
• Infrastructure factors:
– Meet & Pass Planning Point Spacing (MPPPS)
– Signal Spacing (SS)
– Percent Double Track (% DT)
• Traffic and operating factors:
Traffic Peaking Factor (TPF)
Priority Probability (PP)
Speed Ratio (SR)
Average Speed (AS)
Track Outages (TOs)
Temporary Slow Orders (TSOs)
Infrastructure Factors
• Meet & Pass Planning Point Spacing (MPPPS)
– The average spacing of locations used to meet or overtake
trains, namely siding spacing.
– Locations: bi-directional, mixed priority & different speed
– MPPPS for a subdivision is computed as:
Infrastructure Factors
• Signal spacing (SS)
Infrastructure factors
• Percent Double Track (% DT)
– Double or multiple track segments have a significant impact
on a track’s ability to move trains
– Line capacity rises quickly with properly spaced sections of
double track
– Allow for running meets & more than one train in a segment
– %DT is calculated as the ratio of double track vs. the length of
the subdivision
Single Track Bi-directional Running
Meet delay
REES Module 6
Double Track Directional Running
REES Module 6
Bi-directional Running Reduces Capacity
REES Module 6
Directional Running After Merger
REES Module 6
Traffic Factors
• Traffic Peaking Factor (TPF)
– The concentration of traffic within a short time frame (4 hours),
often called bunching or peaking
– Significant impact on capacity
– Result in traffic levels higher than the subdivision could reliably
– When this occurs the effects are felt for a considerable time into
the future as the system recovers from the overload
– TPF is calculated as the ratio between the maximum number of
trains dispatched in a 4-hour period vs. the average number of
trains for that time length
Traffic Peaking Factors Cont.
• Traffic Peaking Factor(TPF)
– Evaluate the amount of traffic on the entire length of the
subdivision at one time
– Designed to be non-directional & include trains in both
– Account for the interaction between opposing trains and the
conflict involving overtaking trains
Traffic Factors
• Priority Probability (PP)
– Dictate which trains will experience delay
– Train priorities decrease capacity
– Priority trains are given preferential treatment over lower
priority trains, which result in increased delays
– The greater the number of priority classes, the less capacity is
REES Module 6
Traffic Factors
• Speed Ratio (SR)
– Besides PP, speed ratio is another parameter reflecting the
traffic mix over the subdivision
– The speed differential between trains can increase delay on a
subdivision by generating overtakes and holding trains in
yards to avoid overtaking on line
Traffic Factors
• Average Speed(AS)
– A significant parameter affecting capacity is average train
– The higher the train speed the lower the delay and transit time
REES Module 6
Options to increase line capacity
• Operations options:
Increase average speed
Reduce traffic peaking
Reduce the variability in speed
Reduce number of meets & passes
• Infrastructure options:
– Line (links):
– Reduce traffic control block length
– Add or lengthen passing sidings
– Additional tracks
– Junctions (nodes):
– Improve junction design
– Grade separation
Operations options should generally be considered first because they are typically
less expensive and faster to implement than new infrastructure
REES Module 6

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