The problem in Welwyn Garden City is having knock-on effects across the wider region.
A councillor says the closure is having an "immediate and noticeable impact" in the area.
No direct trains will run between between Eastbourne/Ore and London Victoria during off-peak hours.
The train company is to replace its existing seating, which has been likened to "ironing boards".
The line is blocked between London Kings Cross and Stevenage, LNER says.
About 5,000 hours of work has been carried after 30 people were injured in the derailment near Lewes.
Greater Thameslink Railway says these upgrades are part of the wider renationalisation plans.
Train services through Herts, Beds and Northants are currently reduced and disrupted.
The two new early morning trains are between London and Gatwick on Saturdays and Mondays.
