user: You are a structured reasoning expert tasked with solving problems step by step. 
You first think about the reasoning process in the mind and then provide the answer. The reasoning process and answer should be enclosed within <think> </think> and <answer> </answer> tags, respectively, i.e., <think> reasoning process here </think> <answer> answer here </answer>. 
Given a question and the associated knowledge graph triplets (entity, relation, entity), you are asked to answer the question with these triplets and your own knowledge. 
You can break down the original question into several sub-questions and explore the given knowledge graph to solve them step-by-step. If there are multiple possible answers, choose one as the final answer. 
After thinking, when you finally reach a conclusion, clearly state the answer within <answer> </answer> tags, i.e., <answer> answer here </answer>. 
Question: find electric system whose locomotive class is sbb-cff-ffs re 420.? 
Knowledge Triplets: SJ Dm3, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 18, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DRG Class E 18, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SNCB Class 28, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DRG Class ET 85, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 420, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SNCB Class 16, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SNCB Class 18, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Rc, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
m.019h, type.permission.controls, m.0wj41nd
NSB El 9, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Ma, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 6, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DR Class 250, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Da, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 420, rail.locomotive_class.uic_classification, Bo′Bo′
DB Class E 40, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 103, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 8, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DBAG Class 425, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
CFL Class 4000, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 151, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 2, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ U, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 7, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DBAG Class 128, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
m.0wj41nd, dataworld.gardening_hint.last_referenced_by, SBB-CFF-FFS Re 420
SJ F, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
ASEA Rz, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DBAG Class 423, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 120, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class E 41, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 420, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 4, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 1, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Pa, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
15 kV AC railway electrification, rail.electric_system.locomotive_class, SBB-CFF-FFS Re 420
DRG Class ET 91, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ D, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 11, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 460, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 14, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 17, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Mg, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DBAG Class 422, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 5, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DR Class 243, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 620, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
m.0wj41nd, type.object.permission, m.019h
NSB El 15, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ O, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SNCF Class BB 37000, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DRG Class E 91, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DRG Class E 77, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DBAG Class 145 and 146, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 420, rail.electric_system.locomotive_class, 15 kV AC railway electrification
SJ Rb, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SBB-CFF-FFS Re 420, rail.electric_locomotive_class.electric_system_s, 15 kV AC railway electrification
DB Class E 10, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 12, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 3, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 111, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
NSB El 16, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Ra, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DRG Class E 16, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SNCF Class Z 26500, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
EuroSprinter, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
DB Class 101, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification
SJ Rm, base.train.electric_train_class.electric_system_s, 15 kV AC railway electrification

Answer: 15 kV AC railway electrification