Servo has a long history in the railroad industry as one of the premiere producers of hot box detectors. Founded in 1946, their thermal sensing technology quickly proved valuable to railroads for detecting hot boxes.

The first Servo hotbox detector was installed on the Chesapeake & Ohio in Norge, VA in 1956. The C&O has started investigating the use of pyrometers to detect hot boxes on passing trains, and reached out to various companies (including Servo) to help develop such a device; Servo began this product development in 1955.

Most of Servo's defect detector models were not of the talker variety. Models prior to the System 9000 simply printed results remotely at a tower, dispatch office, or elsewhere - any defects found would then be relayed by radio to the train crew. In the case of Conrail, Servo's units were so predominate that the clerks whose job it was to check print-outs for defects were known as Servo Clerks.

Servo detectors were modular in nature, comprised of an equipment rack with various components that could be added or upgraded as time went on. Many older Servo models not factory-equipped with a talker were retrofitted with such units as the Devtronics Mini Analyzer (MA1B), Devtronics Mini Talker (MT2A), or Servo's own ServoTalk. This makes it difficult to know what exactly is in the bungalow based on the voice alone.

In 1994, Servo Corporation sold their Transportation Division, comprised of their defect detector line, to Harmon Industries.

Servo 7621 (SERVOSAFE)
The original SERVOSAFE model from the late 1950s that ran on vacuum tubes & transistors. The first reference to the SERVOSAFE name appeared in 1958, although the first Servo detector was installed on the Chesapeake & Ohio in 1956.

Servo 7707 (SERVOSAFE)
An updated version of the Servo 7621 introduced in 1962.

Servo 8808 (SERVOSAFE II)
The first solid state detector introduced by Servo in 1969.

Servo 8909 (SERVOSCAN)
The successor to the 8808, introduced in 1976.

Servo 9909 (SERVOTRIM)
Servo's first microprocessor-based defect detector was revolutionary when introduced in 1981.

Servo 9000 (SYSTEM 9000)
The System 9000 evolved out the solid state ServoTalk, and was a microprocessor-controlled, modular design that allowed railroads to add features as they pleased by simply installing additional cards into the chassis. The System 9000 was Servo's first model factory-equipped with a radio talker. Production of the Servo 9000 started in 1986 and ended in December 2002.

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ServoTalk
In order to "give a voice" to defect detectors, Servo introduced the ServoTalk in 1977, which utilized cassette tapes to transmit audio results over the radio. Starting in 1984, ServoTalk became solid state and started to employ the familiar System 9000 voice. The solid state ServoTalk chassis would evolve into the modular System 9000.