...
if(file_exists("tsave.txt")) 
{ 
	
int index;
var filehandle = file_open_write("tsave.txt");
total_vertices = ent_status(terrain_level1,0);
CONTACT* c;

while(index <= total_vertices)
{
c = ent_getvertex(terrain_level1,NULL,index);  
file_var_write(filehandle,c.z);
index ++;
}
	
file_close(filehandle); // the file now contains "123456.789 1.414 "
		
}
...

if(file_exists("tsave.txt"))
{ 
		
var i, file_data, file_handle;
file_handle = file_open_read("tsave.txt");
CONTACT* c;	
		
if(file_handle) // Check if the file exist, if not, the file handle is going to be 0
{
for(i=0; i <= total_vertices; i++) // for loops don't need a condition, this will work fine
{

file_data = file_var_read(file_handle);
c = ent_getvertex(terrain_level1,NULL,i);
c.v = NULL;
c.z = file_data;
ent_setvertex(terrain_level1,c,i);
}
file_close(file_handle);
}		
}