INTRODUCTION ARUP has provided Cooling the Tube Programme (CTP) with tunnel ventilation modelling for the Sub-Surface Line (SSL) section of the London Underground. The model includes about 8.3 km (13.3 miles) of double-track District and Circle line tunnels and eleven stations from west of Sloane Square to east of Whitechapel Stations. London Underground's Circle line runs along the alignment of the oldest underground railway in the world. Built with steam trains in mind@ the tunnels have historically been quite cool and well ventilated by the blow holes used to relieve the steam. These blow holes now function as natural vent openings to reduce tunnel temperatures. A Subway Environment Simulation (SES) model was developed for the existing trains and future upgraded rolling stock running on the southern portion of the SSL to estimate thermal conditions under normal and congested operations. The model was validated by comparison to present day conditions. The benchmarked model could then be applied to future train operations to estimate the air temperatures in future@ identify the warmer sections@ and if needed@ propose cooling measures to mitigate the high temperatures encountered. The simulations were aimed at predicting the peak air temperatures at 5 P.M. peak operation on a relatively hot July afternoon in the selected portion of SSL. This paper presents selected key modelling issues relating to the SSL and the specific techniques applied to tackle these. A validation or benchmarking process matching results to temperature monitoring data was used to tune these techniques for the current case. This gave more confidence in the predictions of the modelling for future train operations. The main issues on the SSL are: ? Bi-directional tunnels ? Ground water movement ? Future air-conditioned rolling stock The paper discusses the benchmarking process and some key sensitivities@ but does not cover the modelling of the system upgrades except for the impact of air-conditioning.