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