The comedian and Strictly Come Dancing star will voice all scheduled announcements on the trains between London and Edinburgh, Glasgow, Aberdeen, Inverness and Fort William.
A new Caledonian Sleeper fleet is due to launch in May, having been delayed from April 2018 and again in October.
She said: “I spend almost as much time on the Caledonian Sleeper as I do in my own house and so it was a great pleasure to give voice to the new trains.
“I look forward to being woken up by myself. Although it might give me a fright occasionally.”
Earlier this year, 16 new carriages were trialled from Scotland to London Euston ahead of the rollout.
They will provide a hotel-style key card entry system, more accessible rooms, charging panels and Wi-Fi throughout the train.
Room options include a double bed with an en-suite, solo or twin club rooms with an en-suite, a classic room with a solo or twin bed, as well as comfort seats and rooms for passengers with reduced mobility.
Ryan Flaherty, managing director of Serco, which operates the overnight service, said: “Susan is the perfect fit as the voice of Caledonian Sleeper.
“Like us, she has a real sense of adventure and is completely at home travelling around Scotland.
“Our ambition with our new trains is to deliver a timeless experience for guests and I know Susan is thrilled to be part of such a huge change to the service.
“We’re firmly on the final countdown now and I cannot wait for guests to step on board and see the new Caledonian Sleeper for the first time.”