Rail travellers face the prospect of travel chaos from this week if a threatened national strike goes ahead.
The RMT union has announced two strikes, one for 24 hours from 5pm on Thursday and a second for 48 hours from 5pm on June 9.
Union members are also planning to take industrial action short of strike for a week from Saturday.
Staff involved include those responsible for operating signalling equipment on the rail network.
South West Trains, which runs major London commuter services, warned of a “significant impact” on its services if the action goes ahead.
A planned strike over last week’s bank holiday was suspended after Network Rail tabled a revised pay offer to workers.
But the union rejected a fresh pay offer by Network Rail on Thursday.
Both sides have been talking with conciliation service Acas since Friday in an attempt to avert the strike.