Train services have been suspended in parts of Scotland after an amber weather alert for rain and floods.
Cross-border train operators have also warned passengers not to attempt to travel to Scotland on Saturday.
The Met Office said areas of the Highlands and central Scotland could see up to 180mm (7in) of rain.
ScotRail has cancelled services from Oban, Mallaig and Fort William, from Helensburgh Central and on the Highland Mainline between Perth and Inverness.
The train company said most other lines would be disrupted.
The amber alert is in place from from 03:00 on Saturday until 06:00 on Sunday.
A yellow "be aware" warning for heavy rain also covers most of mainland Scotland, as well as Skye and the Inner Hebrides.
Avanti West Coast said they would not run cross-border trains to Scotland on Saturday, with no services north of Preston.
TransPennine advised customers not to travel on trains from Manchester, Liverpool and Preston to Carlisle, Glasgow and Edinburgh.
And there will be no London North Eastern Railway (LNER) services running north of Newcastle.
Police Scotland urged people to avoid all travel unless absolutely necessary.
A number of roads have been closed due to flooding and landslides, including the A83 between Tarbet and Inverary, the A815 between Dunoon and the A83, the A816 between Lochgilphead and Oban and the A85 at Loch Awe.
Diversions have been put in place. Drivers are being urged to take care on the roads and avoid unnecessary travel.
The Met Office said flooding and fast-flowing rivers and streams could pose a risk to life and damage property.
It said 80-100mm of rain could be expected in most areas, with as much as 150-180mm possible for the wettest spots.
The weather warning comes as other parts of the UK could see temperatures of up to 26C (79F) this weekend.
David Simpson, ScotRail service delivery director, said: "We know the impact that the withdrawal of some train services will have, but it's a necessary step to ensure the safety of our staff and customers due to the severe rainfall.
"As a precautionary measure, speed restrictions will also be in place across most routes in Scotland, which will result in extended journey times.
"Customers are advised that they should travel only when necessary, and for those that do, they should check their journey before travelling."
The West Highland line, which operates in Oban, Mallaig and Fort William, will be suspended.
The Helensburgh Central and Balloch to Dalmuir service will also stop, though services to Glasgow should remain in operation.
Services on the Highland Main Line route between Perth and Inverness will also cease.
Affected routes are due to be running again on Sunday morning.
On the roads, traffic on the A83 at the Rest and Be Thankful in Argyll is being diverted on to the Old Military Road as a precaution. It is expected to reopen on Sunday.
The Scottish Environment Protection Agency (Sepa) has issued 14 flood alerts.
It said affected areas included Angus, Ayrshire, Argyll and Bute, and parts of the Highlands, north-east Scotland and the Scottish Borders.
Flood duty manager Ruth Ellis said: "This is a major rainfall event as we are expecting to see significant prolonged heavy rain throughout Saturday, with further heavy rain in the north on Sunday.
"We're likely to see extensive river and surface water flooding, including property flooding and widespread transport disruption."
The West Highland line is one of the ScotRail routes suspended
Scottish government minister for resilience, Angela Constance, said potential impacts of the amber warning were "severe".
She said: "People need to plan ahead and pay close attention to all travel advice."
Police Scotland head of road policing, Ch Supt Hilary Sloan, said: "Our advice is to plan ahead and consider if your journey is really necessary or if it can be delayed until conditions improve.
"Stopping distances can be at least double on wet roads compared to dry conditions, and spray can reduce driver visibility.
"If you need to travel, please drive to the conditions and take extra time for your journey."
