Theresa May will vow never to allow the UK to become "looser and weaker'' in a speech ahead of a meeting in Scotland with Nicola Sturgeon.
Scotrail Strike Brings Roads Misery
Commuters faced travel misery as the first in a series of planned rail strikes led to an increase in rush-hour congestion.
Traffic Scotland warned traffic was "much heavier than normal this morning due to the rail strike'' and congestion in Edinburgh city centre caused disruption to tram journeys.
Train operator ScotRail estimated about 30% of its services were unable to run, hitting tens of thousands of passengers, due to the 24-hour walk-out in a dispute over driver-only trains.
A total of 15 routes, including Glasgow to Inverness and Edinburgh to Aberdeen, were cancelled while the frequency of other services was cut in half.
An Edinburgh Trams spokesman said there were "significant delays'' caused by city-centre congestion and services were shortened.
The Rail, Maritime and Transport (RMT) union, which is opposed to the extension of driver-only and driver-controlled services, balloted ScotRail conductors last month, saying it had not received the assurances it had sought on the issue from the company.
These included guarantees the safety role of conductors and their role in operating train doors would not be reduced or abolished.
Guards at the RMT backed walkouts by a ratio of 3:1, prompting the union to announce seven days of strikes during June and July.
The first day-long strike got under way first thing on Tuesday after planned peace talks failed on Monday.
General secretary Mick Cash said: "The strike on ScotRail is solid as a rock this morning with our members united and determined in the fight for jobs and safety on Scotland's railways.
"This morning ScotRail communications department have let the cat out of the bag and have made it clear that driver operation of the safety-critical functions on our trains is their core objective on all services. That is what this strike is about. The safety of passengers and staff alike.
"ScotRail, and their Dutch owners Abellio, now need to stop the posturing, the dirty tricks and the ultimatums, and get back round the table for serious and meaningful talks. ''
ScotRail managing director Phil Verster said the strike is "totally needless''.
He said: "The RMT have refused point blank to talk to us about how we modernise and improve Scotland's railway. Instead they have hidden behind a national policy that says that nothing must ever change.
"Tens of thousands of our customers will be disrupted and hundreds of our own people will be hit financially as a result of their intransigence.
"The public will be astonished to hear that this is a strike only about who opens and closes doors on trains, nothing more.
"The RMT are being, at best, disingenuous when they tell people that we are trying to have driver-only trains. We are not.
"We will still schedule a second person on-board when we bring in the amazing new faster, longer, greener electric trains from next year.''
Transport minister Humza Yousaf urged both sides to keep talking as he described the action as "very disappointing''.
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