Definition: Compose a new way to ask the same question. The new question must be asking the exact same thing as the original question does. Without any other information, you should be able to tell that the original question and the new question are the same. Preserve bracketed phrases in your new question. You can change the rest of the question provided the above requirements are satisfied. The new question must have a natural, fluent formulation, as if it were composed by a native English speaker. You will be given three pieces of information to help you create better questions. Those are: "Answer": the answer to the question, e.g., "Honolulu". "Answer Type": the type of the answer, e.g., "Location". "Auxiliary Information" is a detailed description of some concepts mentioned in the question, if any.
Input: original question: Find electric system whose locomotive class is [SBB-CFF-FFS Re 420].
answer: 15 kv ac railway electrification
answer type: Electric system.
auxiliary information: The Re 420, originally Re 4/4·¥µ·¥µ, series are the most common electric locomotives of the Swiss Federal Railways. They are used for passenger services throughout Switzerland alone or in pairs. For freight services, they are sometimes paired with the Re 620, especially in mountainous regions. That pairing is referred to by the term Re 10/10. The Re 430, originally known as the Re 4/4·¥µ·¥µ·¥µ, are a derivative of the Re 420 modified for higher traction but lower speed. The Re 420 locomotives were produced over a period of 21 years, from 1964 to 1985. Re 4/4·¥µ·¥µ·¥µ Subseries:·¥µ·¥µ·¥µ When S√ºdostbahn had the opportunity to buy one of the first batch of 50 Re 4/4·¥µ·¥µ locomotives before delivery, it had the gear modified for higher traction and lower speed for the steep routes of the SOB. This locomotive was delivered as number 41. Based on the SOB experience, the SBB ordered a batch of 20 Re 4/4·¥µ·¥µ·¥µ in 1969 for use on the Gotthard route

The locomotive class [SBB-CFF-FFS Re 420] is used on which electric system?