Why change locomotives at the tunnel portals?

Could an early tunnel train switch from steam outside to electric power inside?

Changing locomotives at the portals is preferred because shutting down and restarting the proposed steam plant would be cumbersome.

A participant proposes a locomotive that uses electricity in the tunnel and steam outside it. The objection is that cooling a steam plant and restarting it would be cumbersome. Exchanging a steam locomotive for an electric one at one portal, then changing back at the other, is offered as the more likely arrangement. That objection assumes the steam plant is shut down for the crossing.