When I am quire far advanced in game Running lots of trains and over 300 vehicles game fails to auto save or manual save giving the below error

Failed to write save data to 'C:\Users\Capns\AppData\Roaming\Captain of Industry\Saves\Strategic Sanctuary\1.save': Win32 IO returned 112. Path: C:\Users\Capns\AppData\Roaming\Captain of Industry\Saves\Strategic Sanctuary\1.tmp.save

Ensure that the game has access rights to the save directory and there is enough free space on the drive. Access can be blocked by antivirus software or by Windows feature called 'Controlled Folder Access'.

at System.IO.FileStream.WriteInternal (System.Byte[] src, System.Int32 offset, System.Int32 count) [0x00077] in <932fbe8ac5a946808aa5d7b95844e18c>:0
at System.IO.FileStream.Write (System.Byte[] array, System.Int32 offset, System.Int32 count) [0x00090] in <932fbe8ac5a946808aa5d7b95844e18c>:0
at Mafi.StreamExtensions.CopyToFast (System.IO.Stream fromStream, System.IO.Stream toStream) [0x00022] in <5fce48de31c24fdeb37c99c71b71bed0>:0
at Mafi.Core.SaveGame.SaveLoadFileUtils.SaveDataWithHeaders (System.UInt64 fileHeader, System.Int32 saveVersion, System.IO.Stream dataStream, System.IO.Stream outputStream, Mafi.Core.SaveGame.SaveCompressionType compression, System.Boolean skipCompressedDataChecksum, System.Boolean skipUncompressedDataChecksum) [0x000fc] in <41ba8435d492474781744046547dbc68>:0
at Mafi.Core.SaveGame.GameSaver.FinishSaveWriteToStream (System.IO.Stream outputStream) [0x0003a] in <41ba8435d492474781744046547dbc68>:0
at Mafi.Core.SaveGame.ASyncSaver.doWork (System.Object , System.ComponentModel.DoWorkEventArgs _) [0x000f8] in <41ba8435d492474781744046547dbc68>:0

According to the error code from the message (Win32 IO code 112, not enough free space on the drive to complete write operation), your C drive is full. Which could very easily be a very bad thing, since that's usually the system drive...

Try emptying the recycle bin.

I confess to having had trouble with a full C: drive before and this was the first thing I checked but I have 15GB space according to treesize

15GB "free space" but write errors for lack of space? Might be reserved space for system stability. You could try defragmenting the drive, as that will tend to consolidate remaining space, but emptying the recycle bin and/or uninstalling large applications- CoI is one of those itself, but something tells me you don't want to remove that one- are your best bet.

Well, that and upgrading to a larger drive, but that's even harder than the above.

Oh, and if you have more drives, you might also be able to change the installation drive of the game (and other games) in Steam, thereby placing the game assets elsewhere and letting you reserve the C drive for savegames and the like. Or you could do a nerdy thing and change which drive %APPDATA% is found on, which would change which drive the savegames go to.

Thanks for the info, I deleted a DB-WAL file that had grown excessively, which doubled my free space and game saves fine now.