As with selecting propulsion systems for buses, RTD has to be eminently practical because, bottom-line, transit service must be reliable, on-time and as fast as practicable to support the greatest potential ridership.

In the DMU vs. EMU discussion the key relevant factor is the relative accelleration/decelleration capability of each of the two propulsion systems. Why is this important? Maximizing ridership involves achieving a balance between reaching the maximum number of potential riders with stations convenient to their home and work locations while keeping the overall run time as low as possible. Because EMUs have more low-end torque, they are able to accelerate faster and therefore, are better able to serve more stops while keeping overall run times lower.

Having only two stops, Denver and Longmont, would provide the fastest end-to-end run time but that fastest time might not attract the maximum ridership because everyone in-between Longmont and Denver would be cut out or would have to travel to one end or the other to get on. The opposite extreme would be to have one stop every half mile of urbanized area – this might result in 40 or 50 stops; I don’t know what that would do to the overall run time but you get my point.

As I recall, seven stops were approved by the FasTracks vote: Longmont, Niwot, Boulder, Louisville, Broomfield, Westminster and Denver. I believe five more have been added since the vote. At the July 9 public hearing RTD said they were likely to drop the added five stops and return to the original, voter approved list due to a failure to anticipate materials inflation rates (even for just the first three years) in their original financing plan.

The consultants involved talk about “rapid” and “exploding” population and job growth in the corridor as a part of the “purpose and need” for the expanded rapid transit lines. Whether we end up starting this rail corridor with seven or twelve stops, we can assume that, over time, as population grows in the area, new stations will need to be added to serve newly developed areas – otherwise we will miss picking up their residents and employees as riders.

Please ask staff to calculate the time savings per stop for EMU over DMU. Then see how much difference that makes for an entire 82 mile run from Denver to Longmont and back with seven stops each way. Then calculate how much difference it makes with the currently planned twelve stops each way. Then run the calculation with 15 each way and 25 each way as potential future scenarios.

There are two considerations that follow – first, how much difference will there be in an individual commuter’s travel time between the DMU and EMU with each of the different number of stations? and second, how much (if at all) will the difference influence the number of vehicles that will be needed to maintain different headways between vehicles: 30-minutes, 15-minutes, 10-minutes, 5-minutes and 3-minutes.
This should give us a clear sense of how important a consideration this is in comparing the two technologies.

We need this basic information to be able to make a rational judgement as to the most practical propulsion technology, both for the start of service and for the future of the corridor.

David Cook