If the “pa” part of “companion” comes from path
It doesn't though, it comes from French compagnon/compaignon and then Latin com (with) + panis (bread). It probably originally meant "someone with whom you share bread (eat together)".
And actually, looking at wiktionary, Old English had a word "ġefēra" (with the same meaning) which is constructed very similarly to "спутник": ge ('with', still the same prefix in german e.g. Gebrüder) + fera ('to go'/'to fare', e.g. in seafaring)
I'm pretty sure the model of train is a proper name and it's named after the satellite. I don't think I would describe any train as a literal "sputnik" of the rails.
Also Russian is full of composite words like that. "Explorer" in russian would be "исследователь" (issledovatel') - ис (completely) + след (trace/footstep) + оват (make, imbue) + ель (he who). Literally it would be "he who makes (places) completely (covered in) footsteps"