Design

All the visuals for the show are produced using computer-generated imagery, and that largely explains the high cost of the production (£4 million) . The first series was produced using LightWave 3D software.

Some critics have pointed out the similarities between Underground Ernie and Thomas the Tank Engine and Friends, in that both series feature anthropomorphic locomotives. Whilst acknowledging a similarity, the producers make it clear that this is very different from Thomas, and is very much based in the twenty first century. Despite being underground metro trains, they often travel overground through the open countryside to various stations, including a seaside resort.

The overall design is inspired by the real London Underground, and Transport for London has allowed the series to use design elements directly taken from their brand, in particular the use of their distinctive Underground logo, or 'roundel' as it is known.

3D Films (who previously animated another series called Wheels on the Bus) is responsible for the animation provided, even though both Joella Productions and 3D Films had a bad relationship during production.

Humans

• Underground Ernie is the eponymous supervisor who owns the International Underground station (voiced by ex footballer and television sports presenter Gary Lineker^[2]).
• Millie (voiced by Emma Weaver) is a computer expert working on the network, and the co-owner of the station.
• Mr. Rails (voiced by Howard Ward) is the maintenance man.
• Dan and Sam (voiced by Chris Jarvis and Tim Whitnall) are Ernie's mischievous nephews, They like to cause a lot of trouble around the international underground.

Trains

The majority of the characters are trains, most of which take their names from the lines of the London Underground network, although in Underground Ernie's world, it is called "International Underground".

• Victoria (voiced by Janet Brown) is the oldest train, and is like a mother to the other trains. She wears a crown, to suggest the link between herself and Queen Victoria, but visually appears more like a caricature of the former Queen, Elizabeth II. Although Victoria is presented as the oldest character in the series, the Victoria line in real life is actually one of the newest lines – only the Jubilee line, opened in 1979, was newer at the time.
• Bakerloo (voiced by Howard Ward) is an eccentric and has a deerstalker hat similar to that of Sherlock Holmes (the Bakerloo line used to terminate at Baker Street, where the fictional detective lived at number 221b) but has a profile resembling that of the Duke of Wellington, the victor of the Battle of Waterloo, after the Waterloo Underground station (Bakerloo is short for Baker Street & Waterloo Railway).
• Hammersmith (voiced by Tim Whitnall) is twin brother to City, a fast and furious train who is a general knowledge "whizz". He wears blue spectacles.
• City (voiced by Chris Jarvis) is Hammersmith's twin brother, and is also fast and furious, but is an expert in mathematics and finances (presumably a reference to the City, where many of the banks and financial businesses in London and the United Kingdom are based). He wears pink spectacles. Hammersmith and City are usually coupled back end to back and travel together as one train (a play on the Hammersmith & City line).
• Jubilee (voiced by Tim Whitnall) is the youngest train, and is into computers, music, internet, emails, games, and all the latest gadgets and technology. He can be mischievous, but is still well liked.
• Circle (voiced by Chris Jarvis) is a "hippy chick"—always helpful and rarely loses her cool.

There are also five international trains, which Millie visits when she goes abroad every weekend. She always bring back exciting stories for Ernie, as well as a souvenir:

• Brooklyn (voiced by Howard Ward) is from New York City, is decked out in the American Stars and Stripes.
• Sydney (voiced by Emma Weaver) is from Australia, always wants to go to the beach.
• Moscow (voiced by Howard Ward) is from Russia. He is strong, fast and likes to clear snow off the tracks. He often uses the wrong word when trying to use various English phrases, such as saying "as fit as a fishcake" instead of "as fit as a fiddle".
• Paris (voiced by Chris Jarvis) is from France. He's snooty, but kind.
• Osaka (voiced by Tim Whitnall) is from Japan, and has a super-speed booster. He is decked out in a Rising Sun decorated bandana.

Toys

In November 2005, Halsall International Ltd. announced that they had secured the rights to produce merchandising linked to Underground Ernie . The products became on sale by September 2006, with a comprehensive range being available in 2007. The use of London Underground names and trains is a deliberate step to lead into merchandising suitable for children's gifts, the revenue from which was shared with Transport for London. Halsall International Ltd has, since 2009, stopped making 'Underground Ernie' models.

From 2007 up until 2009, Bachmann produced an HO scale Underground Ernie range, which was discontinued after the show was taken off air.

DVDs

Warner Vision International released four compilation DVDs of the show, which were later compiled into a boxset forming the complete first series. These releases are now out of print, and are very hard to find.

Underground Ernie charity evenings

As well as Days Out with Ernie, in 2008 and 2009, two Underground Ernie fundraising events were held at the railway which were hosted by The Karen Ingram Foundation With over £3,500 raised on the night at the first event, which raised money for the hospital which cared and looked after Karen Ingram, for whom the events were in memory of. Co creator John Deery also attended the special charity evening, along with his family.

With Eastleigh no longer hosting events involving Days Out with Ernie, a final fundraiser was held at the railway in 2010, however this was not themed to Ernie, due to the railway no longer having the Ernie characters available to use.