Earliest railroad electrification

I'm basically wondering if there are any major impediments to significantly earlier railroad electrification? Sprague's motor does seem to be the first that could realistically be used, but I'm not clear on whether there is any particular reason what he did couldn't be pushed back at least to the 1870s around the time lights started appearing? Just how early could it be technologically reasonable to see someone electrify a streetcar?
 
Werner von Siemens built a demonstration track in Berlin 1n 1879 and then Siemens built the Gross-Lichterfelde Tramway in 1881 which ran until 1931.
In the UK the Volk's Electric Railway was built in Brighton in 1883 and still runs today and the Underground began electrifying in 1890. You might be able to push the Underground date earlier by a little but you'll be hard pressed to be earlier than Siemens.
 
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