How to get more capacity on London’s transport network is a perennial debate. Upgrade signalling systems. Get longer trains. Build Crossrail 2.

But there’s one piece of infrastructure that’s already in place and cuts through the length of the city – and it’s debatable whether we’re making the most of it.

There are two commuter-style riverboat services that travel into the centre of London from either side of the city. There’s another, central only, one that operates between the two Tate galleries. And there’s a river crossing-style passenger ferry between Canary Wharf and the Hilton Docklands, across the river in Rotherhithe. (Did you know that was there? I didn’t.) All of these are operated by Thames Clippers.

Transport for London (TfL) itself runs the free passenger and vehicle ferry between Woolwich and North Woolwich (OK, pedants; Briggs Marine and Environmental operates it, but it’s a TfL service). Then there are various tourist services with running historical commentary, which commuters with hangovers and a day full of meetings are only going to use if they have a lot of disposable income or are trapped in an episode of The Good Place.

Freight takes up more space on the Thames. Construction of the super sewer will use the river to transport materials in an attempt to reduce HGVs on the road. The now-defunct Garden Bridge planned to do similar, in what was possibly the only sensible part of that whole project.

Yet there’s room for more. The Port of London Authority (PLA) notes that increased development near the river, notably at Nine Elms, brings opportunities for more commuter services. Indeed, a new pier at Battersea will be served by Thames Clippers in the near future. By 2035, the PLA wants 20m commuter and tourist trips a year, almost double the current figure.

So where are those trips going to come from?

There’s one service that seems obvious, but doesn’t currently exist: a route to City Airport. It’s on the river, right? Thames Clippers serves a pier further east, and the Woolwich Ferry already docks close by. Why not add a boat from the west stopping at North Woolwich?

Boring practical answers are: it’s actually a 15 minute bus ride from North Woolwich pier to the airport, and the DLR already goes there. But the DLR is only useful if you’re already in the City; if you’re around the West End it would be a lot easier to hop on a boat and whizz down the river.

Ticketing is an added complication to any expansion of services. London’s Travelcard and pay as you go systems work with a series of concentric ring zones, radiating outwards from zone 1, the most central, and most expensive: you simply buy a ticket for the zones you want to travel in.

But Thames Clippers don’t use the same zone boundaries. They divide the river into three zones: the west zone covers roughly the same are as zone 2 on that side of London (I suspect the actual dividing line is slightly further out than the zone 2/3 boundary on the tube map, but no matter). But the central zone goes all the way out to Canary Wharf, deep into the non-central Zone 2, and the east zone extends to Woolwich Arsenal, zone 4 if you took the DLR.

A map of Thames Clipper services. Click to expand.

None of this really matters, of course, because the Clippers don’t use TfL’s fares anyway. Your Travelcard isn’t valid (though it will get you a third off a standard fare). You can use pay as you go on Oyster, but that fare doesn’t count towards your daily cap. And it’s expensive: one journey in the central zone costs £6.30 on Oyster or buying online or with an app. The daily pay as you go cap, the maximum you can spend on all other forms of transport around zone 1, is just £6.60.

A month’s pass to commute between, say, Wandsworth and Blackfriars, costs £188.15. By way of comparison, a monthly Zone 1 & 2 Travelcard costs £126.80, you can use it on more than one mode of transport and journey times are roughly comparable. Although, to be fair, one of these journeys is probably a lot more pleasant than the other.

At any rate, we may be getting towards an explanation of why river services aren’t more popular.

One interesting footnote about the Thames Clippers fare policy is that it is partly controlled by City Hall. Peruse the fares chart, and you’ll spot some weird anomalies in the fares between West-and-Central and East-and-Central. Turns out that the RB6 route between Putney and Canary Wharf is operated under contract from TfL, and a mayoral directive sets the fares.

Thames Clippers was awarded RB6 after previous operators couldn’t make the route work commercially. Since 2013, the company has increased the number of passenger journeys and added more boats to the service. In theory, other river services could be brought back under TfL’s control – but in practice, while Thames Clippers is making a profit, there’s no reason to do so.

It’s unsurprising that TfL has no desire to absorb the full impact of the costs of river routes. Given that TfL has recently decided it can’t afford the planned upgrade to the Northern and Jubilee lines, there’s no way it’s going to take on another expensive service.

This is a shame, as making river transport an integrated part of the Travelcard system is an obvious way of encouraging use. Stockholm includes ferries in its own travelcard, and Sydney includes public ferries in its daily and weekly capping system.

But perhaps comparing London to these cities is unfair. After all, if you ask Transport NSW’s website how to get from central Sydney to, say, Manly, it tells you to get the ferry – it’s just the easiest option. Similarly, when your city is a collection of islands like Stockholm, it makes sense for ferries to be a seamless part of the system.

But in London, it’s faster to get from Westminster to Putney on the District Line. The RB1 route is mostly connected up by the District or Jubilee lines. Apart from the bit around Chelsea, which is an odd transport desert (and will stay that way, if residents succeed in overturning plans for a Crossrail 2 station), river services feel like an optional extra.

The PLA doesn’t envisage capacity issues restricting growth in commuter services, and though Thames Clippers is adding boats, it’s likely that commercial viability will be the big constraint in making the Thames a practical piece of transport infrastructure.

Unless, just possibly, you look east.

It’s easy to think of Thames-based public transport as just being for the area within Greater London. But Thames Clippers is looking at running a service from Gravesend to Embankment with a calling point at Canary Wharf – and did a four day trial in September. The whole journey takes 1 hour 10 minutes, a favourable comparison with the hour it takes Southeastern to get to Charing Cross.

Ticketing may not be as big an issue, depending on the type of season ticket a Kentish commuter chooses: currently, a Southeastern-only monthly ticket costs £252.30. There’s an option to pay £385.60 if you want a TfL travelcard on top, but if your home and social life is based in Gravesend, would you bother, or just go use Pay As You Go to move around London? Given the choice between a scenic commute on the river or playing sardines on Southeastern, this feels like a no-brainer.

The PLA believes there is potential for new piers at Barking Riverside, Thamesmead, Purfleet, Erith, Greenhithe and Grays. So instead of thinking of the Thames as a way to unlock London’s transport potential, it might be more useful to look towards Essex and Kent, and use the river to relieve the area’s creaking rail infrastructure.

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