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Railway Enigma
Railway variant of Enigma K (A27) - wanted item

During WWII, the Germans used a special Enigma machine for the German Railway (Reichsbahn). It was basically a standard Enigma K with rewired wheels and a rewired UKW. Furthermore, the position of the turnover notches of wheels I and III were swapped [1]. Enigma traffic from the German Reichsbahn was first encountered by the codebreakers at Bletchley Park on 25 July 1940 and all messages were decrypted until the traffic ceased a month later, on 27 August 1940.

According to internal BP reports, Colonel Tiltman was responsible for breaking Railway Enigma during this period. Early in the next year, on 23 January 1941, railway traffic was intercepted again, originating from Eastern Europe, Russia and the Balkans. Two weeks later, on 7 February 1941, the traffic was broken for the first time.

BP named the railway traffic key Rocket, but later renamed it to Rocket I. Although there are no known images of a Railway Enigma, it is most likely that it was a standard Enigma K with rewired wheels and a rewired UKW.
  
Enigma D, which was the design on which Enigma K and the Railway Enigma were based

In his Report on E Operations of GC&CS, US codebreaker W.F. Friedman claimed that Railway Enigma had a movable UKW, but this is highly unlikely. First of all, as the machine only has a single notch on each wheel, the UKW would hardly ever step if it could move at all. Secondly, the only machines known to have a movable UKW are the Zählwerk Enigma and Enigma G, both of which have multiple turnover notches that cause frequent stepping of the UKW. It is likely that Friedman meant that the UKW was settable, which is the case with Enigma D and Enigma K.

Rocket I
After the first successful breaks in 1941, the railway traffic key was named Rocket. It was mainly used on railway traffic networks in Eastern Europe, Russia and the Balkans. It was later renamed to Rocket I in order to discriminate it from other railway networks.

Breaking Rocket I was relatively easy for BP, and the network provided good intelligence about production and movement of supplies. Nevertheless, problems with Rocket I were reported on 19 September 1944, and it wasn't before 28 October that BP gained entry into the traffic again. The blackout was apparently caused by eccentricities in the cribs during this period [1].

The problems with Rocket I illustrate that even a standard Enigma K, without the Army's plugboard (Steckerbrett) could be hard to break if the contents of the messages were less predictable. It also shows that the Bombes were virtually useless without good cribs.

Rocket II and III
In September 1942, a similar key appeared for Western Europe. The new key was called Rocket II and was only broken once when it was using the Rocket I machines. Apparently, the traffic consisted of practicing messages only. Judging from the characteristics of these messages, the (practicing) traffic continued until May 1944, after which a new key, and possible another machine, was introduced. They new key was called Rocket III and remained unbroken for quite some time. For a long time it was unclear whether Enigma was used at all.

After capturing some key sheets in August 1944, some of the older messages were decrypted. It became clear that Enigma was used, but that the contents of these messages were sufficiently 'obscure' so that cribs could not be used easily. It is most likely that Rocket II and III used the (military) Service Enigma (Enigma I). Rocket II was later renamed to Blunderbuss.

Wiring
The table below shows the wiring for the standard Railway Enigma (Rocket I). Only wheels I, II and III were used. Note that the turnover notches are the same as on commercial Enigma K, but that the position of the notches of wheels I and III has been swapped. This swapping was relatively easy, as the notch (gap) is part of a removable ring that is held in place by four bolts.

Wheel ABCDEFGHIJKLMNOPQRSTUVWXYZ Notch Turnover #
ETW QWERTZUIOASDFGHJKPYXCVBNML      
I JGDQOXUSCAMIFRVTPNEWKBLZYH V N 1
II NTZPSFBOKMWRCJDIVLAEYUXHGQ M E 1
III JVIUBHTCDYAKEQZPOSGXNRMWFL G Y 1
UKW QYHOGNECVPUZTFDJAXWMKISRBL      

References
  1. David Hamer, Geoff Sullivan and Frode Weierud
    Enigma Variations: An Extended Family of Machines

    Cryptologia, July 1998, Volume XXII, Number 3.
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Crypto Museum. Created: Saturday 18 September 2010. Last changed: Monday, 12 June 2017 - 20:23 CET.
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