What’s on a locomotive cab roof?

What’s on a locomotive cab roof? A cluttered battle for available space. But that wasn’t always the case. Decades ago, the roof of a locomotive cab was pretty bare. A majority of them had little more than an air horn and a single radio antenna mounted on it. The antenna was connected to the voice radio inside the cab, allowing the employees inside to talk to dispatchers, operators, crewmembers in the caboose or employees along the right of way. From the introduction of voice radios in the railroad industry to today, railroads have slowly added more antennas for the increasing amount of data being exchanged. The explosion of wireless technology has transformed the industry even more, with locomotives going from a single antenna on its roof more than a half-century ago to more than a dozen antennas atop the newest locomotives built today.

The voice antenna is still there, but today it has to co-exist with the myriad of antennas to support systems such as Positive Train Control, GPS, and cellular data. Each antenna must work alongside the others around it, with spacing and ground plane requirements that need to be addressed, as well as the use of filters between the antenna and its equipment to prevent radio interference. To make matters more complex, certain systems such as PTC have multiple antennas for multiple data paths on different systems for redundancy to increase safety and reduce downtime. And those multiple PTC antennas on the roof? They also have their own redundancy in the form of a duplicate antenna for each one in case of failure of the primary antenna.

The rise of air conditioning installed on regular cab locomotives has forced the relocation of them to other places such as underneath the cab floor or in front of the engineer on the walkway outside the cab. Other railroads will simply build platforms next to or above existing roof-mounted air conditioners for new antennas to reside.

Here are the typical antennas found on the roof of a modern PTC-equipped locomotive. Note that many of these have redundant antennas, typically for PTC usage: Distributed Power, Cellular Modem, 802.11 Wireless, VHF Voice Radio, GPS for locomotive manufacturer use, GPS for PTC use, 802.11 Wireless, 220 MHz PTC radio, and Head of Train Device.

For more on locomotive cab roofs, check out Guss’s article, “Antennas galore.”