What the RAIB investigation will seek to establish

• The level of wheel/rail adhesion present on the approach to Salisbury Tunnel junction.

• The status and performance of the braking, wheel slide protection and sanding systems on train 1L53.

• The behaviour of both trains during and following the collision.

• South Western Railway’s policies relating to low wheel/rail adhesion.

• Network Rail’s policies relating to low wheel/ rail adhesion and how it managed the risk of low adhesion in this area.

• The processes used to assess and control the risk of overrun at signal SY31.

• Any relevant underlying factors, including any actions taken in response to previous safety recommendations.