New Caledonian Sleeper trains to make passenger debut tonight
They will feature the first double beds in the service's 146-year history and the first ensuite showers for nearly a century.
The inaugural service is scheduled to leave Glasgow Central before linking with the Edinburgh portion of the train at Waverley station in the capital and then heading to London.
Another of the new £150 million fleet is due to leave London Euston tonight, with portions for Edinburgh and Glasgow being divided en route.
The milestone will be celebrated with an event at Glasgow Central expected to be attended by transport secretary Michael Matheson, Scotland Secretary David Mundell and Rupert Soames, chief executive of Serco, which won the 15-year Caledonian Sleeper contract in 2015.
Serco said the "revitalised" trains would launch a "magical new era in travel".
The firm will tonight also unveil an "inspirational" marketing campaign "which encourages guests to Dream Big".
It said: "Caledonian Sleeper is more than just a train ride - it is a world of possibilities".
The new fleet of 75 carriages are a year late, having been delayed twice from April last year by design and manufacturing hold-ups.
They will also run on the service's routes between Aberdeen, Inverness and Fort William and London, but a date has still to be confirmed.
The current sleeping carriages were introduced in the 1980s, and lounge cars in the 1970s, with their toilets at the ends of coaches.
The new trains have free wi-fi, charging sockets and hotel-style keycard entry to cabins.
Prices for basic bunk bed cabins - or "Classic Rooms" - start at £140 for solo use or £170 for two sharing.
This compares to £85 for sharing with a stranger in the past, which has been scrapped.
Club Rooms, with bunk beds, basin and ensuite toilet/shower wet room, and breakfast included, are priced from £205 for one person and £250 for two.
Caledonian Doubles, which have double beds and the same facilities as Club Rooms, cost from £335 for one person and £400 for a couple.
Double and twin rooms with space for wheelchairs are also available.
The price of seats remains at from £45, which include reading lights and overhead lockable storage cabinets for valuables.
A revolution is also promised in the lounge car, which can accommodate 34 people compared to 18 in the old trains.
It includes a "saw tooth" table down the side for solo travellers.
The catering is expected to be transformed by a state-of-the-art galley replacing the previous kitchen, which just had microwaves for food preparation.