Sources cite different completion dates depending on what counts as completion. Through rail service from Moscow to the Pacific existed by 1904, but it ran via the Chinese Eastern Railway across Manchuria rather than entirely within Russian territory. The all Russian route, avoiding Chinese territory via the Amur line, was not finished until 1916. Both dates are defensible depending on whether the question is asked about through service or an entirely domestic route.
What would resolve this Nothing further needs discovering; the dispute is resolved by stating both dates with what each one measures, which this atlas does in the infrastructure entitys own completion year note rather than picking one silently.
OpenRailway historyWikipedia: Trans-Siberian Railway