This document summarizes the wheel damage issue that occurred on a light rail system in 2008. An investigation found that inadequate brake inspection allowed wheel defects to occur fleet-wide. The underlying problem was determined to be poor integration of the train braking systems with the automatic train protection system. A temporary fix was implemented that energized track brakes on ATP overspeed commands, which increased braking to meet performance standards and eliminated further wheel damage.

1. Karl W. Berger, P.E.
DCM, Inc.
Centreville, VA
IEEE/ASME National Capital Land Transportation Committee
December 8, 2009

2. Long term inadequate inspection allowed
fleet-wide wheel defects.
Investigation revealed underlying problem
caused by poor integration of car braking
systems with Automatic Train Protection
(ATP).
A temporary fix completely eliminated
wheel damage.
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3. Opened in 1992 as a single-track line.
Central Line + 2 branches = 30 miles.
Serves 33,600 passengers per day.
53 large LRVs by ABB (Bombardier).
Originally equipped with Automatic Block
Signals (ABS) and magnetic Trip Stops.
Several collisions revealed deficiencies
with Trip Stops.
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4. Progressively upgraded Central Line
between 2004 and 2006.
Replaced ABS with Cab Signaling with
Automatic Train Protection.
Removed wayside signals and trip stops.
Reduced headways from 17 minutes to 8
minutes.
Improved service reliability.
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5.  Wayside signal system impresses each
track block with a speed codes.
 Speed code inductively coupled to lead
vehicle in train.
 Operator normally controls train below
received speed code.
 Audible alert if operator exceeds speed.
 Automatic Train Protection applies brakes
if operator or vehicle fails to respond.
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6. Guaranteed Emergency Brake Rate
(GEBR) of 1.5mphps (0.67 m/s2)
implemented with vital systems.
GEBR achieved with single-point failures
in braking systems.
Signal block lengths designed to include
adjustment for grade and curvature.
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7. Condition Cab Signal Command LRV Response
Speed <
Speed Code – 1.5 mph
Normal Operate Normally
Speed =
Speed Code – 1.5 mph
Propulsion Disable Inhibit Propulsion
Speed >
Speed Code + 0.5 mph
Penalty 1 (Overspeed) Apply Full Service
Braking
Penalty 1 Active
AND
Decel < 1.5 mphps
for > 3 seconds
Penalty 2
(Brake Assurance)
Apply Emergency
Brakes (vital)
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8. APPROACH8 – Used at interlockings.
Immediate Penalty 2 Emergency Brake on
Loss of Speed Code or Overspeed.
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9. Constraints due to nearly obsolete car
control computer.
Added redundancy to remove single-point
failures in friction and track brake systems.
Added two emergency magnet valves for
ATP Emergency Brake applications.
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10. April 2008 – broken wheel found on LRV
prior to entering service.
Fleet inspection found many wheels with
excessive damage and flats.
Investigation started.
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11. The fall leaf season intensified the
problem.
Northern half of system shut down in early
November 2008.
Only 7 cars operational by mid-November.
Intensified research into problem and
solutions.
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13. Causes:
• Inadequate slide control in service braking
• No slide control in Emergency Braking
• Problems induced by other defects.
Distribution:
• Brake Assurance – General 36%
• Brake Assurance – Greasy Rail 22%
• APPROACH8 anomaly 21%
• Unknown/Unaccounted 14%
• Operator commanded 7%
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14. Component failures in signal cabins
caused momentary loss of speed code at
interlockings. (APPROCH8 Anomaly)
Excessive grease on curves reduced
adhesions.
Accounted for 43% of Emergency Brake
flats.
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15. Inadequate slide control in service braking
– typically 0.8 to 1.2 mphps during slide.
No slide control in Emergency Braking.
Higher adhesion demand on unpowered
center trucks.
Maintenance was not refilling sand boxes.
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17. LRVs equipped with six electromagnetic
track brakes (one pair per truck) .
Each pair provides ≈ 0.8 mphps.
Sliding wheels + Track Brakes > 1.5mphps
Not fail safe – power required for
actuation.
Cannot be used to meet GEBR.
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19. Energize one pair of track brakes on ATP
overspeed (Penalty 1).
Track brakes add ≈0.8 mphps to full
service brakes. GEBR satisfied with sliding
wheels.
Vitality provided by existing Emergency
Brake response.
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20. Added two diodes and an On/Off switch.
Effectiveness verified by test.
Vitality of ATP confirmed by Failure Mode
Effects and Criticality Analysis.
Entire fleet retrofitted and tested in one
week.
Full service over entire system restored by
the end of November 2008.
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22. Wheel flatting eliminated.
Center Truck track brakes shoe life
reduced to 3 months.
ATP software modification added
discrimination and restored acceptable
shoe life.
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23. Improve dynamic brake slide control.
Improve friction brake slide control.
Replace car control computer.
Improve ATP integration.
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