They were once a regular sight on the Valleys Lines when trains were out of service - and now carriages and locomotives look set to be coming back.
The "Class 37" diesel electric locomotives, which are more than 50 years old, are going to be coming back to pull commuter services in south Wales and fill the gap as some of the trains currently being used are refurbished.
The old engines were initially built between 1960 and 1965 for InterCity services but in recent decades have only been used to pull passenger trains.
They have been hired by Transport for Wales as part of a number of things being done in the short term to increase capacity as the operator waits for newer rolling stock.
Transport for Wales boss James Price has said the following will all be happening on the Valley lines:
Four sets of locomotive-pulled carriages
The "Class 37" trains have been hired until the end of the year and are a locomotive, with four carriages attached.
Those being added will mean some of the existing stock - one by one will be removed to be refurbished.
These have 220 seats so there will be an increase in seating capacity on some services, according to Transport for Wales.
The current plan is to also continue to operate these through the autumn - when weather tends to cause problems.
The first will come into use on Monday, and in coming weeks another one.
Five former Great Western Railway trains are coming in
Five ex-GWR trains - known as a Class 153 - will be brought onto various lines across the Wales and Borders network so other trains can help with peak morning and afternoon services on the Rhymney Valley line.
It now means seven of the eight morning peak services into Cardiff and all eight afternoon peak services out of Cardiff will be made up of four carriage sets.
A previously flood-damaged train is coming back
A long-stopped Class 150 is going back into service after repair of flood damage.
'More modern, refurbished' trains being introduced
Once the rental of the Class 37 train hire ends, new bi-mode Class 769 trains will come in.
They are expected on the Rhymney line from September.
"The Class 769s are planned to be in use, alongside class 150s from the existing fleet, until all trains in the Rhymney Valley Line are replaced with brand new, tri-mode trains from 2023.
"These innovative trains, which will be able to run on diesel (only off the core Valley Lines) overhead electric and battery power, will offer a more frequent service which provides greater capacity and faster journeys than the current serice."
Mr Price's letter was in response to questions by Hefin David AM at the Assembly's economy committee.
He has been giving evidence to the committee since Transport for Wales took over train services in October 2018.
He said that since changes to the timetable in May, there is an increase of 11% in carriages in the morning peak against the last 10 weeks, and up 14% for the afternoon peak.
