London Road is closing for investigations following the two-train crash.
The road is completely closed from St Mark's roundabout in both directions and a diversion is in place.
Wiltshire Police are asking drivers to avoid the road.
The force said: "We've closed London Road from the St Mark's roundabout as enquiries continue following the rail collision last night.
"We would ask motorists and members of the public to avoid this area if possible and follow diversions which are now in place.
"This work is being led by British Transport Police. It is difficult to predict how long closures will be in place for – this will be dictated by the investigation."
Yesterday evening, two trains collided between Salisbury and Andover.
Read more:
- Two-train crash between Salisbury and Andover - all we know so far
- Live updates after train crash in Fisherton Tunnel
At 6.46pm, two trains carrying passengers were involved in a crash in Fisherton Tunnel.
British Transport Police said that no one died in the crash.
According to Salisbury NHS Foundation Trust, 13 casualties were treated at Salisbury District Hospital, with four casualties admitted, one patient discharged and three remain as inpatients and are in a stable condition.
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