The Piccadilly line’s new air conditioned trains

Original Article ↗ Hacker News Discussion ↗

New air‑conditioned trains for London’s Piccadilly line are being welcomed as a major comfort upgrade, but they also highlight the engineering and financial constraints of modernising the 19th‑century Underground. Commenters focus on how designers “budgeted” heat by using more efficient motors, braking and lighter construction to add AC without further warming already overheated deep tunnels, and why cooling stations and tunnels remains far harder than cooling carriages. There is broader debate over line‑upgrade priorities, the shelving of planned signalling improvements that would have boosted capacity, and the UK’s wider infrastructure funding and governance challenges.

Line Upgrade Priorities (Piccadilly vs Central/Bakerloo/Victoria)

  • Many ask why Central or Bakerloo weren’t upgraded first, given Central’s heat and noise.
  • Responses:
    • Central’s trains are newer (1992 stock) and undergoing major refurbishment (including motor upgrades), so full replacement is less urgent.
    • Bakerloo has much lower ridership and overlaps other services; Piccadilly carries roughly twice the passengers and serves Heathrow.
    • Piccadilly offered the largest modeled capacity gain (around 60%), so was prioritized.
    • Some note Victoria already had a major modernisation with cooler trains around 2009.

Engineering Challenge of AC in Deep Tube Tunnels

  • Core problem: deep-level tunnels are small, poorly ventilated, and surrounded by clay that has retained heat over decades.
  • AC moves heat from the carriage into the tunnel and adds extra waste heat; without somewhere to dump that heat, tunnels keep getting hotter.
  • Several point out the constraint is not headroom but waste-heat removal; you “can’t beat thermodynamics.”
  • Subsurface lines have AC more easily due to bigger tunnels and legacy ventilation for steam trains.

“Heat Budget” and Train Design

  • New Piccadilly trains are lighter, more efficient and use regenerative/dynamic braking and LED lighting.
  • They produce less total heat than the old stock; the saved “heat budget” is spent on AC while keeping net tunnel heating roughly unchanged.
  • Some worry the tunnels may still be warming over the long term; whether equilibrium has been reached is described as unclear.

Signalling, Capacity, and Missed Opportunities

  • Original “New Tube for London” concept tied new trains to modern signalling for big frequency gains and additional stops.
  • Covid-era budget cuts reportedly axed or delayed the signalling upgrade; capacity increase is now more modest (e.g. ~24→27 tph plus ~10% more capacity per train).
  • There is debate over whether further resignalling is coming; some say design contracts exist, others highlight huge costs and delays on other lines.
  • Frustration that skipped stations (e.g. Turnham Green) may still not get regular Piccadilly stops despite new stock.

Passenger Comfort, Noise, and Air Quality

  • Strong relief about AC, especially given the Piccadilly’s reputation for extreme heat (“Tandoor”, “Central Heating Line” jokes).
  • Concerns remain about:
    • High noise levels on several lines; some argue track grinding and better maintenance are more urgent than AC.
    • Poor air quality (“dark mist”, black dust), largely from braking and rail wear; regenerative braking should reduce dust somewhat.
    • Dated interiors, small windows, use of fabric moquette seats that trap dirt and potentially bedbugs.

Cooling and Heat Reuse Ideas

  • Numerous speculative ideas: geothermal loops, groundwater cooling, pumping heat to surface district heating, even bringing ice blocks.
  • Some real-world examples mentioned: station cooling projects using boreholes and groundwater, and a scheme that pipes waste heat into local heating networks.
  • Counterpoint: adding shafts or large-scale heat extraction in dense central London is described as technically feasible but extremely expensive and constrained by space and politics.