Image copyright Gareth James/Geograph Image caption GWR are promising a "new, super-fast hourly" train service between Cardiff and London at rush hour

Quicker and more frequent trains are promised between south Wales and London in what is being described as the biggest timetable revamp for 40 years.

Great Western Railway (GWR) aims to shorten Cardiff to London journey times by up to 17 minutes.

Transport for Wales (TfW) has added room for 6,500 more Valley Lines passengers in Sunday's timetable change.

An extra 186 Sunday services will also be added in Wales.

But there were problems on Saturday with train cancellations, blamed on staff shortages and work on the new timetable, with 15 services also affected on Sunday.

"This time of year can always be challenging, particularly on weekends and we're sorry that there have been a number of cancellations on our network," a TfW spokesman said.

"As always, we are very grateful to colleagues who gave up their own time to volunteer to work overtime and rest days."

GWR tweeted that anyone experiencing problems on their trains would be able to claim their ticket price back "if you have a single, return, or weekly ticket" or if their train is "over two hours later than planned".

Image caption Prof Cole said when the Valleys Lines are electrified in three years' time, the amount of services will double

Transport expert Prof Stuart Cole, from the University of South Wales, called it "a major change" - with the fact trains will arrive in Cardiff faster from London meaning services to other parts of Wales will also have to change.

This is so people are not waiting for connecting trains when they arrive.

In terms of putting more services in place on Sundays, he described this as "relatively straightforward".

"The world has changed from the days when there used to be a full service Monday to Friday," he said.

"That was extended to Monday to Saturday, with Sunday left as the Cinderella.

"But there has been the realisation more people want to do shopping in Cardiff or travel as a tourist (on Sundays)."

In terms of Saturday's 15 services cancelled, he said it sounded a lot, but it may have only amounted to one or two trains being out of action.

GWR say the "biggest timetable change on this network since 1976" is to maximise the speed of their new 93-strong fleet of 10-carriage 125mph (201 km/h) Hitachi class 800 trains, which were launched in 2017.

The company, which wants to "unlock the potential" of their new high speed trains, reckons typical journey times on its twice-hourly Cardiff to London service will be shortened by 14 minutes to one hour and 53 minutes.

GWR will also add a new rush hour express service between the two capitals with trains running non-stop between Bristol and Paddington, cutting journey times to one hour and 42 minutes.

Passengers travelling between Cardiff and Portsmouth and Cardiff and Taunton and Exeter can also expect to see train upgrades to increase capacity.

"The timetable change this December will be the biggest since 1976 and will mean faster journeys and more frequent services for passengers," said Mike Gallop, of Network Rail.

Image copyright Transport for Wales Image caption TfW's refurbished fleet will be in use between Cheltenham and Maesteg and Cardiff and Ebbw Vale

Passengers who use Transport for Wales' Cheltenham and Maesteg and Cardiff and Ebbw Vale services will get revamped trains with air conditioning, wi-fi and power sockets.

The old trains from those routes will be moved to often overcrowded Valley Lines services to provide space for up to 6,500 more commuters a week.

TfW says it will also upgrade carriages on trains between north Wales and Manchester and introduce an additional service between Holyhead and Cardiff.

Train bosses have warned passengers to check before they travel in case some of their usual travel arrangements have changed.

TfW's 186 service additions to its Sunday timetable is a 40% increase and it says this will boost tourism and provide "essential links between cities, towns and villages".

A spokesman added: "Over the past few months we have been meticulously planning and preparing for the new timetable by recruiting additional staff and we're confident that customers will see a real change as we introduce the additional services including extra capacity across our network."

The additional Sunday services include: