The inspiration for the modern style of sympathetic rebuilding is widely held to be Liverpool Street in the mid-1980s. Before then the complete destruction of the old Euston, the burying of Sunderland and Birmingham, and the unfortunate juxtaposition of the Manchester Arena over Manchester Victoria’s through platforms could not be said to be sympathetic – but they did all have one thing in common with their modern relatives: the stations still had to serve passengers throughout the reconstruction process.

Most building contractors would raise their eyebrows if you suggested moving millions of passengers around their building sites, but that is what Network Rail and its partners must do to bring such sights as the new western concourse at King’s Cross to fruition.

The final package of work at King’s Cross – the demolition of the 1970s southern concourse and the creation of King’s Cross Square – is due for completion in autumn 2013, and the work of re-routing passengers around and through the building work will continue until then. How they do so, and which walking routes are used between platforms, concourse and onward connections, is a matter of considerable importance to the project team.

Kink's Cross worksite

Passengers leaving platforms 0-8, under the original – but now fully restored – train shed roof, currently have to follow a passageway between hoardings funnelling them towards the Underground station, while others peel off left and right to other destinations. This is no accident. In fact, so important was access to the Tube, the worksite was cut in half to allow for the route straight out to it.

Around 70 per cent of all passengers arriving at King’s Cross continue their journeys by Underground. Lessons learned in previous projects show that diverting passengers is rather like diverting a river; you have to go with the flow.

While the contractors would have preferred one work site, instead of two, the impossibility of closing an access point used by a large proportion of the station’s passengers meant the channel had to be created. Given the tidal nature of travel to and from the Tube, there has never been the need to have flows separated, as the weight of passengers does that naturally during the morning and evening peaks.

Interestingly, the nature of the passengers using a station can have an effect on how effective signage can be during periods of redevelopment. Leisure travellers tend to be less aware of where to go, and will follow signage much more attentively than commuters, who are programmed to follow the same routes they have always done. They still adapt quickly however, with the most effective form of communication being people, preceded by early warnings in the form of signage on platforms and announcements on trains.

When the new western concourse opened at King’s Cross in March 2012, teams were employed to engage with the travelling public personally, using giant foam hands to direct them into the new building. Toc’s played their role too, with on-train announcements, posters, leaflets, web information and that most modern form of communication, Twitter.

London Bridge development

Another important lesson from the past is that passengers adapt better to one big change to their route in or around a station than many little ones. For that reason, the Nottingham station redevelopment project set up new passenger flows in October 2012, which will stay in place until the project is complete in early 2014.

The former main entrance has been largely closed off, leaving one access route for those requiring step-free access. Temporary ticket facilities have been created at the former milk dock along the side of the station and a temporary footbridge erected. These arrangements will remain until the redeveloped station is revealed in all its glory, with a new glazed concourse in the porte cochere, and a brand new concourse to the south.

The high water mark for judging passenger flows was the Olympic and Paralympic Games last year, where Olympic ticketing information was combined with event locations and commuter flows to draw up station ‘hot-spots’ and a colour coded map for passengers, showing what times should be avoided and where.

While much important information is still gleaned through people standing at stations with counters, pedestrian flow modelling software can also help design alternative flows, with a variety of different programmes available to help.

The representation of passengers can be virtual reality or just red dots depending on the programme used, and they can be programmed to behave in different ways; with a certain percentage carrying heavy luggage, some with visual impairments and others just lost.

Interestingly, modelling passenger flows is a much easier task on the Underground, thanks to the data gleaned from Oyster cards, which can track individual journeys and habits over years and years. When smart-ticketing is eventually installed on the national rail network, similar data will help develop stations of the future.

Project teams also work closely with station staff, fire safety officers and passenger groups in designing routes, as there is no substitute for experience of a station and the types of people who use it.

One aspect of stations that can now be worked on without impacting passenger flows is the roof. The restoration of the main train shed at King’s Cross presented a particular challenge, as the project team not only had to replace the glazing, but also strip 32 layers of paint – most of it lead based – while working above people’s heads.

Scaffolding was erected to run on ‘rails’ fitted to the station walls and abutments many metres above track level, so it could be moved along as the work progressed without the need to impede passengers with scaffolding at ground level. Both sides of the roof were enclosed so the paint dust could be safely extracted, and the enclosed space was also heated so the paint could be applied and dried in the winter.

It’s the kind of solution that passengers are not always aware of, barring the gloomier train shed while the work is carried out, but that means our most important stations can function fully while simultaneously being taken apart and improved.

Now the biggest challenge that Network Rail faces at its stations lies a mile or so further south, at London Bridge.

Where the project team at King’s Cross was able to create a new platform (platform 0) in the old cab road, the project to rebuild London Bridge has no such luxury. Taking two or three platforms out of use at a time will allow the team to gradually dismantle the current station and create a new one, with greater capacity and better passenger flows (including a street-level concourse bigger than the pitch at Wembley).

But to do it will require changing passenger flows and rerouting trains until 2018. It’s a challenge, but one that Network Rail and its contractors are well used to facing.